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This base::RefCountedMemory subclass owns a
base::ReadOnlySharedMemoryMapping instance.

To demonstrate the usefulness of this new class, convert PdfCompositor
from mojo::ScopedSharedBufferHandle to base::ReadOnlySharedMemoryRegion.
Use base::RefCountedSharedMemoryMapping to wrap memory regions and avoid
data copies.

While changing PdfCompositor, turn some DCHECKs into actual checks,
since there is no guarantee shared memory options will always succeed.

BUG=826213

Change-Id: I3135235ea350f58c83606e530b33ed6aa7be3cb5
Reviewed-on: https://chromium-review.googlesource.com/989429Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Wei Li <weili@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553255}