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This CL teaches the cc scheduler about the asynchronous painting of
PaintWorklets. While PaintWorklets are being painted, the pending tree
cannot be activated (as it is not fully painted). As such, we must
delay activation (even via aborting the frame) until the painting has
finished.

Note that at this time the asynchronous painting has not been landed (as
it depends on this scheduling behavior). Support will be added in
upcoming patches; see the prototype:
https://chromium-review.googlesource.com/c/chromium/src/+/1577635

Bug: 907897
Change-Id: I7d9ec466c57fbe3f8088c5ff6bf6faa140bd4b83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649656
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669114}