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/fonts/platform/windows/font_list.rs

@@ -14,7 +14,7 @@ use style::values::computed::{FontStyle as StyleFontStyle, FontWeight as StyleFo
 use style::values::specified::font::FontStretchKeyword;
 
 use crate::{
-    EmojiPresentationPreference, FallbackFontSelectionOptions, FontTemplate,
+    EmojiPresentationPreference, FallbackFontSelectionOptions, FontIdentifier, FontTemplate,
     FontTemplateDescriptor, LowercaseFontFamilyName,
 };
 
@@ -78,9 +78,10 @@ where
             let local_font_identifier = LocalFontIdentifier {
                 font_descriptor: Arc::new(font.to_descriptor()),
             };
-            callback(FontTemplate::new_for_local_font(
-                local_font_identifier,
+            callback(FontTemplate::new(
+                FontIdentifier::Local(local_font_identifier),
                 template_descriptor,
+                None,
             ))
         }
     }