Simplify mojo_base shared memory region support

Changes the SharedMemoryRegion types in mojo_base mojom to use a
handle<shared_buffer> as their internal representation. This simplifies
the traits and makes these region types usable in Java and JS.

Bug: 826213
Change-Id: Id515f61f34b85c655b70d45d53302e873547a83a
Reviewed-on: https://chromium-review.googlesource.com/1059008
Commit-Queue: Ken Rockot <rockot@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559207}
4 files changed