fonts: Simplify FontContext in two ways that affect the unit test (#33541)

This is done by no longer forwarding compositor-bound messages through
SystemFontService and making `FontContext` non-generic:

- Messages from the `FontContext` to the `Compositor` no longer need to be
  forwarded through the `SystemFontService`. Instead send these messages
  directly through the script IPC channel to the `Compositor`.

- Instead of adding a mock `SystemFontServiceProxy`, simply implement a
  mock `SystemFontService` on the other side of an IPC channel in the
  `font_context` unit test. This allows making `FontContext`
  non-generic, greatly simplifying the code. The extra complexity moves
  into the unit test.

These changes necessitate adding a new kind of `FontIdentifier`,
`FontIdentifier::Mock` due to the fact that local fonts have
platform-specific identifiers. This avoids having to pretend like the
system font service can have web fonts -- which was always a bit of a
hack.

These two changes are combined into one PR because they both require
extensive and similar chages in the font_context unit test which
dependended on the details of both of them.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/canvas/canvas_data.rs

@@ -11,7 +11,7 @@ use euclid::default::{Box2D, Point2D, Rect, Size2D, Transform2D, Vector2D};
 use euclid::point2;
 use fonts::{
     ByteIndex, FontBaseline, FontContext, FontGroup, FontMetrics, FontRef, GlyphInfo, GlyphStore,
-    ShapingFlags, ShapingOptions, SystemFontServiceProxy, LAST_RESORT_GLYPH_ADVANCE,
+    ShapingFlags, ShapingOptions, LAST_RESORT_GLYPH_ADVANCE,
 };
 use ipc_channel::ipc::{IpcSender, IpcSharedMemory};
 use log::{debug, warn};
@@ -434,7 +434,7 @@ pub struct CanvasData<'a> {
     old_image_key: Option<ImageKey>,
     /// An old webrender image key that can be deleted when the current epoch ends.
     very_old_image_key: Option<ImageKey>,
-    font_context: Arc<FontContext<SystemFontServiceProxy>>,
+    font_context: Arc<FontContext>,
 }
 
 fn create_backend() -> Box<dyn Backend> {
@@ -446,7 +446,7 @@ impl<'a> CanvasData<'a> {
         size: Size2D<u64>,
         webrender_api: Box<dyn WebrenderApi>,
         antialias: AntialiasMode,
-        font_context: Arc<FontContext<SystemFontServiceProxy>>,
+        font_context: Arc<FontContext>,
     ) -> CanvasData<'a> {
         let backend = create_backend();
         let draw_target = backend.create_drawtarget(size);