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This patch serializes PaintShaders of kImage type (backed by
PaintImages).

R=enne@chromium.org, ericrk@chromium.org

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: I46384a879aac0b4bff7321b3ad20aac97dd8e069
Reviewed-on: https://chromium-review.googlesource.com/865879
Commit-Queue: vmpstr <vmpstr@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529159}