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Updates platform handle wrapping/unwrapping API signatures to be more
extensible for a stable ABI, and adds support to shared memory region
wrapping/unwrapping for writable regions, which may be represented by
multiple platform handles on some platforms.

TBR=bajones@chromium.org

Bug: 826213,842037
Change-Id: Ia0e5da7b7e0f4c41853eb3a8d4a12da47071dd8c
Reviewed-on: https://chromium-review.googlesource.com/1058440Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559165}