media: remove base::SharedMemoryHandle usage in v4l2 encoder

This replaces a use of the legacy UnalignedSharedMemory ctor
taking a SharedMemoryHandle with the current ctor taking a
PlatformSharedMemoryRegion.

Bug: 849207


Change-Id: Iea24ebdcd941cf2fa97e19cf2aeac1a18f9773d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1697602
Commit-Queue: Matthew Cary (CET) <mattcary@chromium.org>
Reviewed-by: Ricky Liang <jcliang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681740}