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/flow/inline/text_run.rs

@@ -8,8 +8,7 @@ use std::ops::Range;
 use app_units::Au;
 use base::text::is_bidi_control;
 use fonts::{
-    FontContext, FontRef, GlyphRun, ShapingFlags, ShapingOptions, SystemFontServiceProxy,
-    LAST_RESORT_GLYPH_ADVANCE,
+    FontContext, FontRef, GlyphRun, ShapingFlags, ShapingOptions, LAST_RESORT_GLYPH_ADVANCE,
 };
 use fonts_traits::ByteIndex;
 use log::warn;
@@ -342,7 +341,7 @@ impl TextRun {
     pub(super) fn segment_and_shape(
         &mut self,
         formatting_context_text: &str,
-        font_context: &FontContext<SystemFontServiceProxy>,
+        font_context: &FontContext,
         linebreaker: &mut LineBreaker,
         font_cache: &mut Vec<FontKeyAndMetrics>,
         bidi_info: &BidiInfo,
@@ -410,7 +409,7 @@ impl TextRun {
     fn segment_text_by_font(
         &mut self,
         formatting_context_text: &str,
-        font_context: &FontContext<SystemFontServiceProxy>,
+        font_context: &FontContext,
         font_cache: &mut Vec<FontKeyAndMetrics>,
         bidi_info: &BidiInfo,
     ) -> Vec<(TextRunSegment, FontRef)> {
@@ -556,7 +555,7 @@ pub(super) fn add_or_get_font(font: &FontRef, ifc_fonts: &mut Vec<FontKeyAndMetr
 
 pub(super) fn get_font_for_first_font_for_style(
     style: &ComputedValues,
-    font_context: &FontContext<SystemFontServiceProxy>,
+    font_context: &FontContext,
 ) -> Option<FontRef> {
     let font = font_context
         .font_group(style.clone_font())