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/layout_2020/context.rs

@@ -6,7 +6,7 @@ use std::sync::Arc;
 
 use base::id::PipelineId;
 use fnv::FnvHashMap;
-use fonts::{FontContext, SystemFontServiceProxy};
+use fonts::FontContext;
 use net_traits::image_cache::{
     ImageCache, ImageCacheResult, ImageOrMetadataAvailable, UsePlaceholder,
 };
@@ -27,7 +27,7 @@ pub struct LayoutContext<'a> {
     pub style_context: SharedStyleContext<'a>,
 
     /// A FontContext to be used during layout.
-    pub font_context: Arc<FontContext<SystemFontServiceProxy>>,
+    pub font_context: Arc<FontContext>,
 
     /// Reference to the script thread image cache.
     pub image_cache: Arc<dyn ImageCache>,