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Also, consolidate all PaintOp validation into new IsValid
functions.

Bug: 749023, 750010
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I1f5b695fe71c50d5ae126937c15a8957285a5487
Reviewed-on: https://chromium-review.googlesource.com/590749Reviewed-by: Vladimir Levin <vmpstr@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Commit-Queue: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491171}