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When serializing a SkTextBlob, skia flattens it into allocated memory
which is then copied into PaintOpWriter's memory. Change this to
directly serialize into the writer's memory.

R=enne@chromium.org, reed@google.com
BUG:821584

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I644a02aba1b8f5b204596eaf2b04d8763a9cce37
Reviewed-on: https://chromium-review.googlesource.com/961274Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548599}