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If the canvas has a mode fixed (acceleration or not acceleration),
ShouldAccelerate should not change it even though the hint is set
to the other side.
This CR fixes that when the canvas is accelerated will stay
accelerated.
Changing unittest and fixing a series of tests that were affected
as a side effect of blurriness.
For the browsertests I'm making some test to activate pixels (as now
these test are using accelerated canvas and we need to generate the
pixels).

TBR=

Bug: 947532, 973448, 977179
Change-Id: Ib81e7ce4bc0a8ab360df8deeb523f4e70e363706
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1700113
Commit-Queue: Juanmi Huertas <juanmihd@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677392}