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This CL restricts the number of outbound requests to the paint preview
compositor for processing bitmaps to 4. This should limit the peak
memory usage and might reduce OOM crashes.

This CL also makes it possible to cancel queued requests. As a result,
when flinging or zooming where many requests are made at once, it is
possible to clear pending requests for tiles that exit the viewport
before the request is served.

Bug: 1142545
Change-Id: I8f4859c61e5ffd1fa017dd840fac57fe8deb79ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2497825
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821418}