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This CL moves the paint of the background color off the main
thread. Here is the design doc:
https://docs.google.com/document/d/1usCnwWs8HsH5FU_185q6MsrZehFmpl5QgbbB4pvHIjI/edit

In particular, we follow the infra of paint worklet.
When main thread paints, we create a PaintDeferredImage,
which is a place holder that does nothing. And then we
create a NativePaintWorkletInput that captures necessary
info and pass it to the compositor thread. After commit,
we ask the compositor thread to use the information from
the NativePaintWorkletInput and paint the background color.
Once the paint is done, we activate the pending tree.

There is no need to add new test, as long as the current
test are passing because this CL should not cause any
behavior change.

Bug: 1139004
Change-Id: Id487ca3c8511e620bcc652e75199f893227a50e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466140
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828858}