Previously, the `ImageCache` would inform consumers of changes to image
availability by messaging an IPC channel. This isn't great, because it
requires serialization and deserialization. Nowadays, `ImageCache`s are
always owned by the same `Global` that uses them. This change replaces
the IPC channel with callback that implements `Send`.
For the case of normal HTML document images, the results are still sent
over the pre-existing Crossbeam channel that was connected via an IPC
route. A followup change might eliminate that channel entirely.
Testing: This should not change observable behavior so is covered by
existing tests.
Fixes: #24338.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Mukilan Thiyagarajan <mukilan@igalia.com>