This change addresses the bug where images unable to load when the
reload button in the UI is clicked repeatedly. Before this fix, it was
possible to use SharedImageRequests across multiple documents. However,
when the document that initiated the request is gone, tasks scheduled
on the event loop remain in the fetching state because the originating
document is no longer active. Furthermore, another reason to prohibit
the sharing of image requests across documents is that the "Origin"
header in an image request is dependent on the document.
If a function that captures a GC-allocated object is owned by another
GC-allocated object, it is more preferable to use JS::HeapFunction.
This is because JS::HeapFunction is visited, unlike introducing a new
heap root as JS::SafeFunction does.
Callbacks registered within the SharedImageRequest can be removed after
the request has been completed. This resolves the GC memory leak issue
that occurs due to a cyclic dependency, where the callback captures the
image request while being owned by the image request at the same time.
This allows to partially solve the problem of cyclic dependency between
HTMLImageElement and SharedImageRequest that prevents all image
elements from being deallocated.
As it turns out, making everyone piggyback on HTML::ImageRequest had
some major flaws, as HTMLImageElement may decide to abort an ongoing
fetch or wipe out image data, even when someone else is using the same
image request.
To avoid this issue, this patch introduces SharedImageRequest, and then
implements ImageRequest on top of that.
Other clients of the ImageRequest API are moved to SharedImageRequest
as well, and ImageRequest is now only used by HTMLImageElement.
This fixes an issue with image data disappearing and leading to asserts
and/or visually absent images.
