cc/paint: Add a paint op helper with a ToString functionality.

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/764390
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516057}

• cc/BUILD.gn[diff]
• cc/paint/paint_op_buffer.h[diff]
• cc/paint/paint_op_helper_unittest.cc[Added - diff]
• cc/test/paint_op_helper.h[Added - diff]