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This change upgrades RTCVideoEncoder output_buffers_ from
base::SharedMemory to the new API base::UnsafeSharedMemoryRegion.
The only functional difference is that the region is mapped for
each BitstreamBufferReady() rather than only once when the shared
memory is allocated.

Bug: 849207
Change-Id: I6404b395d3b676f677ff9e9d6ccab3ff206e4099
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1697642
Commit-Queue: Matthew Cary (CET) <mattcary@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683965}