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In the process, convert the buffer_types.mojom to handle<platform>. This
also adds a legacy IPC::ParamTraits<ScopedHandle> to support sending
GpuMemoryBufferHandle over legacy IPC.

Tbr: flackr@chromium.org
Bug: 863011, 710376
Change-Id: I087d9d83edc729b512dbb79806a0b4b2db2b31bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2064609
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745586}