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Add an UnsafeSharedMemoryRegion constructor for BitstreamBuffer.
While BitstreamBuffer continues to represent its shared memory
internally by base::subtle::PlatformSharedMemoryRegion due to
being used sometimes read-only and sometimes writable, this
constructor makes the bitstream buffer easier to use in the common
case of client/tests which create a buffer backed by unsafe
(writable) shared memory.

Bug: 849207
Change-Id: I271bcd389e936ab2e7207bb2684fc6bcd8c2d529
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1695264
Commit-Queue: Matthew Cary (CET) <mattcary@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681431}