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This patch adds serialization to PaintShaders, with the exception of
PaintImage and PaintRecord.

R=enne@chromium.org

Bug: 737629
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Idbaf512f2f0c12df8edd6bcd459af0ddaaff4126
Reviewed-on: https://chromium-review.googlesource.com/590675
Commit-Queue: Vladimir Levin <vmpstr@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491889}