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Make SharedBitmap return an UnguessableToken as the shared bitmap-based
tracing id, if the SharedBitmap is in fact backed by shared memory for
cross-process use. This means ClientSharedBitmapManager always does
and ServerSharedBitmapManager returns null if the bitmap was allocated
inside the ServerSharedBitmapManager.

In ServerSharedBitmapManager memory dumps, if a bitmap is shared memory
backed, it dumps the |mapped_id()| which is the UnguessableToken.
Otherwise, it dumps the SharedBitmapGUID from the SharedBitmapId.

In ResourceProvider, if the SharedBitmap has an UnguessableToken for
its shared memory backing, it dumps that, else uses the SharedBitmapID
to make a GUID.

In ResourcePool, the same thing is done as ResourceProvider.

Thus, all child-process bitmaps will be dumped via the shared memory
mapped_id() which is an UnguessableToken. And all service-process
bitmaps are dumped via the SharedBitmapGUID.

R=ericrk@chromium.org

Bug: 730660
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: Id4b1c20350a139d36df13743d012756181937c6e
Reviewed-on: https://chromium-review.googlesource.com/935428
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541275}