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/font_cache_thread.rs

@@ -484,6 +484,7 @@ impl FontSource for FontCacheThread {
                     identifier: serialized_font_template.identifier,
                     descriptor: serialized_font_template.descriptor.clone(),
                     data: font_data.map(Arc::new),
+                    stylesheet: None,
                 }))
             })
             .collect()