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/inline.rs

@@ -11,7 +11,7 @@ use app_units::{Au, MIN_AU};
 use base::print_tree::PrintTree;
 use bitflags::bitflags;
 use euclid::default::{Point2D, Rect, Size2D};
-use fonts::FontMetrics;
+use fonts::{FontContext, FontMetrics};
 use log::debug;
 use range::{int_range_index, Range, RangeIndex};
 use script_layout_interface::wrapper_traits::PseudoElementType;
@@ -33,7 +33,7 @@ use style::values::specified::text::TextOverflowSide;
 use unicode_bidi as bidi;
 
 use crate::block::AbsoluteAssignBSizesTraversal;
-use crate::context::{LayoutContext, LayoutFontContext};
+use crate::context::LayoutContext;
 use crate::display_list::items::{DisplayListSection, OpaqueNode};
 use crate::display_list::{
     BorderPaintingMode, DisplayListBuildState, StackingContextCollectionState,
@@ -1239,7 +1239,7 @@ impl InlineFlow {
     /// `style` is the style of the block.
     pub fn minimum_line_metrics(
         &self,
-        font_context: &LayoutFontContext,
+        font_context: &FontContext,
         style: &ComputedValues,
     ) -> LineMetrics {
         InlineFlow::minimum_line_metrics_for_fragments(
@@ -1255,7 +1255,7 @@ impl InlineFlow {
     /// `style` is the style of the block that these fragments belong to.
     pub fn minimum_line_metrics_for_fragments(
         fragments: &[Fragment],
-        font_context: &LayoutFontContext,
+        font_context: &FontContext,
         style: &ComputedValues,
     ) -> LineMetrics {
         // As a special case, if this flow contains only hypothetical fragments, then the entire