fonts: Remove web fonts when their stylsheet is removed (#32346)

This is the first part of ensuring that unused fonts do not leak. This
change makes it so that when a stylesheet is removed, the corresponding
web fonts are removed from the `FontContext`.

Note: WebRender assets are still leaked, which was the situation before
for all fonts. A followup change will fix this issue.

Fixes #15139.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Mukilan Thiyagarajan <mukilan@igalia.com>

components/gfx/tests/font_context.rs

@@ -24,6 +24,7 @@ use servo_atoms::Atom;
 use servo_url::ServoUrl;
 use style::properties::longhands::font_variant_caps::computed_value::T as FontVariantCaps;
 use style::properties::style_structs::Font as FontStyleStruct;
+use style::stylesheets::Stylesheet;
 use style::values::computed::font::{
     FamilyName, FontFamily, FontFamilyList, FontFamilyNameSyntax, FontSize, FontStretch, FontStyle,
     FontWeight, SingleFontFamily,
@@ -85,6 +86,7 @@ impl MockFontCacheThread {
                 Self::url_for_font_name(name),
                 std::sync::Arc::new(data),
                 &CSSFontFaceDescriptors::new(name),
+                None,
             )
             .unwrap(),
         );