Add test for FontContext/FontGroup functionality

Unfortunately, this required quite a bit of changes to the non-test
code. That's because FontContext depends on a FontCacheThread, which in
turn depends on a CoreResourceThread and therefore lots of other data
structures.

It seemed like it would be very difficult to instantiate a FontContext
as it was, and even if we could it seems like overkill to have all these
data structures present for a relatively focused test.

Therefore, I created a FontSource trait which represents the interface
which FontContext uses to talk to FontCacheThread. FontCacheThread then
implements FontSource. Then, in the test, we can create a dummy
implementation of FontSource rather than using FontCacheThread.

This actually has the advantage that we can make our dummy
implementation behave in certain specific way which are useful for
testing, for example it can count the number of times
find_font_template() is called, which helps us verify that
caching/lazy-loading is working as intended.

components/gfx/font_cache_thread.rs

@@ -3,6 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use app_units::Au;
+use font_context::FontSource;
 use font_template::{FontTemplate, FontTemplateDescriptor};
 use fontsan;
 use ipc_channel::ipc::{self, IpcReceiver, IpcSender};
@@ -30,7 +31,7 @@ use style::values::computed::font::{SingleFontFamily, FamilyName};
 use webrender_api;
 
 /// A list of font templates that make up a given font family.
-struct FontTemplates {
+pub struct FontTemplates {
     templates: Vec<FontTemplate>,
 }
 
@@ -41,14 +42,14 @@ pub struct FontTemplateInfo {
 }
 
 impl FontTemplates {
-    fn new() -> FontTemplates {
+    pub fn new() -> FontTemplates {
         FontTemplates {
             templates: vec!(),
         }
     }
 
     /// Find a font in this family that matches a given descriptor.
-    fn find_font_for_style(&mut self, desc: &FontTemplateDescriptor, fctx: &FontContextHandle)
+    pub fn find_font_for_style(&mut self, desc: &FontTemplateDescriptor, fctx: &FontContextHandle)
                                -> Option<Arc<FontTemplateData>> {
         // TODO(Issue #189): optimize lookup for
         // regular/bold/italic/bolditalic with fixed offsets and a
@@ -89,7 +90,7 @@ impl FontTemplates {
         None
     }
 
-    fn add_template(&mut self, identifier: Atom, maybe_data: Option<Vec<u8>>) {
+    pub fn add_template(&mut self, identifier: Atom, maybe_data: Option<Vec<u8>>) {
         for template in &self.templates {
             if *template.identifier() == identifier {
                 return;
@@ -414,8 +415,8 @@ impl FontCache {
     }
 }
 
-/// The public interface to the font cache thread, used exclusively by
-/// the per-thread/thread FontContext structures.
+/// The public interface to the font cache thread, used by per-thread `FontContext` instances (via
+/// the `FontSource` trait), and also by layout.
 #[derive(Clone, Debug, Deserialize, Serialize)]
 pub struct FontCacheThread {
     chan: IpcSender<Command>,
@@ -453,7 +454,31 @@ impl FontCacheThread {
         }
     }
 
-    pub fn find_font_template(&self, family: SingleFontFamily, desc: FontTemplateDescriptor)
+    pub fn add_web_font(&self, family: FamilyName, sources: EffectiveSources, sender: IpcSender<()>) {
+        self.chan.send(Command::AddWebFont(LowercaseString::new(&family.name), sources, sender)).unwrap();
+    }
+
+    pub fn exit(&self) {
+        let (response_chan, response_port) = ipc::channel().unwrap();
+        self.chan.send(Command::Exit(response_chan)).expect("Couldn't send FontCacheThread exit message");
+        response_port.recv().expect("Couldn't receive FontCacheThread reply");
+    }
+}
+
+impl FontSource for FontCacheThread {
+    fn get_font_instance(&mut self, key: webrender_api::FontKey, size: Au) -> webrender_api::FontInstanceKey {
+        let (response_chan, response_port) =
+            ipc::channel().expect("failed to create IPC channel");
+        self.chan.send(Command::GetFontInstance(key, size, response_chan))
+            .expect("failed to send message to font cache thread");
+
+        let instance_key = response_port.recv()
+            .expect("failed to receive response to font request");
+
+        instance_key
+    }
+
+    fn find_font_template(&mut self, family: SingleFontFamily, desc: FontTemplateDescriptor)
                                                 -> Option<FontTemplateInfo> {
         let (response_chan, response_port) =
             ipc::channel().expect("failed to create IPC channel");
@@ -470,7 +495,7 @@ impl FontCacheThread {
         }
     }
 
-    pub fn last_resort_font_template(&self, desc: FontTemplateDescriptor)
+    fn last_resort_font_template(&mut self, desc: FontTemplateDescriptor)
                                                 -> FontTemplateInfo {
         let (response_chan, response_port) =
             ipc::channel().expect("failed to create IPC channel");
@@ -486,31 +511,8 @@ impl FontCacheThread {
             }
         }
     }
-
-    pub fn add_web_font(&self, family: FamilyName, sources: EffectiveSources, sender: IpcSender<()>) {
-        self.chan.send(Command::AddWebFont(LowercaseString::new(&family.name), sources, sender)).unwrap();
-    }
-
-    pub fn get_font_instance(&self, key: webrender_api::FontKey, size: Au) -> webrender_api::FontInstanceKey {
-        let (response_chan, response_port) =
-            ipc::channel().expect("failed to create IPC channel");
-        self.chan.send(Command::GetFontInstance(key, size, response_chan))
-            .expect("failed to send message to font cache thread");
-
-        let instance_key = response_port.recv()
-            .expect("failed to receive response to font request");
-
-        instance_key
-    }
-
-    pub fn exit(&self) {
-        let (response_chan, response_port) = ipc::channel().unwrap();
-        self.chan.send(Command::Exit(response_chan)).expect("Couldn't send FontCacheThread exit message");
-        response_port.recv().expect("Couldn't receive FontCacheThread reply");
-    }
 }
 
-
 #[derive(Clone, Debug, Deserialize, Eq, Hash, PartialEq, Serialize)]
 pub struct LowercaseString {
     inner: String,