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This patch adds a helper class that can stringify PaintOps. This is
done in a best effort manner to help with debugging. Also adds
corresponding unittests.

R=enne@chromium.org

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ic5003365251fa40710e7a79ccf3934096d9f7d6a
Reviewed-on: https://chromium-review.googlesource.com/764390Reviewed-by: enne <enne@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516057}