Clean up HTMLImageElement::areas
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Improve behaviour of image elements that perform multiple requests
This addresses cases where image elements end up making multiple requests, as well as makes the element respond to additional relevant mutations that trigger updating the image data.
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Remove the redundant tracked loads for the underlying image request; the
load blockers present in the ImageRequest structure perform the same
duty with better accuracy.
In support of this goal, the layout thread collects information about
CSS images that are missing image data and hands it off to the script
thread after layout completes. The script thread stores a list of
nodes that will need to be reflowed after the associated network
request is complete. The script thread ensures that the nodes are
not GCed while a request is ongoing, which the layout thread is
incapable of guaranteeing.
The image cache's API has also been redesigned in support of this
work. No network requests are made by the new image cache, since it
does not possess the document-specific information necessary to
initiate them. Instead, there is now a single, synchronous
query operation that optionally reserves a slot when a cache
entry for a URL cannot be found. This reserved slot is then
the responsibility of the queryer to populate with the contents
of the network response for the URL once it is complete. Any
subsequent queries for the same URL will be informed that the
response is pending until that occurs.
The changes to layout also remove the synchronous image loading
code path, which means that reftests now test the same code
that non-test binaries execute. The decision to take a screenshot
now considers whether there are any outstanding image
requests for layout in order to avoid intermittent failures in
reftests that use CSS images.
Per https://html.spec.whatwg.org/multipage/embedded-content.html#dom-img-width:
The IDL attributes width and height must return the rendered width and
height of the image, in CSS pixels, if the image is being rendered, and is
being rendered to a visual medium; or else the density-corrected intrinsic
width and height of the image, in CSS pixels, if the image has intrinsic
dimensions and is available but not being rendered to a visual medium; or
else 0, if the image is not available or does not have intrinsic dimensions.
Honor SERVO_ENABLE_DEBUG_ASSERTIONS on the build machines.
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As part of #13127.
cc @aneeshusa
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