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/shared/compositing/lib.rs

@@ -141,7 +141,7 @@ pub struct CompositionPipeline {
 pub enum ForwardedToCompositorMsg {
     Layout(ScriptToCompositorMsg),
     Net(NetToCompositorMsg),
-    Font(FontToCompositorMsg),
+    SystemFontService(FontToCompositorMsg),
     Canvas(CanvasToCompositorMsg),
 }
 
@@ -150,7 +150,9 @@ impl Debug for ForwardedToCompositorMsg {
         match self {
             ForwardedToCompositorMsg::Layout(_) => write!(f, "Layout(ScriptToCompositorMsg)"),
             ForwardedToCompositorMsg::Net(_) => write!(f, "Net(NetToCompositorMsg)"),
-            ForwardedToCompositorMsg::Font(_) => write!(f, "Font(FontToCompositorMsg)"),
+            ForwardedToCompositorMsg::SystemFontService(_) => {
+                write!(f, "SystemFontService(FontToCompositorMsg)")
+            },
             ForwardedToCompositorMsg::Canvas(_) => write!(f, "Canvas(CanvasToCompositorMsg)"),
         }
     }