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The broad strokes behind this CL is that TracedValue now can be
supplied with a factory function, which is used to override its
backing implementation.

When Perfetto is enabled, this is used to let TracedValue write
directly to a proto which can then be memcpy'd into the Perfetto
shared buffer, rather than being written into a base::Pickle which
is then serialized as a JSON strong.

Dependent on these CLs landing and rolled into //third_party/perfetto:
https://android-review.googlesource.com/c/platform/external/perfetto/+/792813
https://android-review.googlesource.com/c/platform/external/perfetto/+/792814

TBR=csharrison@chromium.org

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I2bfa0cd7398a0ac4788c56d5be13fe951dc56770
Reviewed-on: https://chromium-review.googlesource.com/c/1263354Reviewed-by: oysteine <oysteine@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603768}