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This CL adds a base::SharedMemoryHandle getter to gpu::BufferBacking.
The getter will be used by gpu::BufferBacking users when OnMemoryDump is
called to create base::SharedMemory dumps.

This is part of work for exposing SharedMemory usages on tracing
memory-infra dumps.

Design doc: https://docs.google.com/document/d/16Mi5_puxKgQ-9IX7ANfbji0WiqBCK3yrnJ0Qi2wa3w0/edit#heading=h.lofdeze1a2hr

Note: This can conflict with https://codereview.chromium.org/2912723002/

BUG=604726
TEST=gpu_unittests --gtest_filter=Buffer.*
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2909003002
Cr-Commit-Position: refs/heads/master@{#475837}