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Adds mojo/public/cpp/platform to the public library. This includes
a new PlatformHandle type, a thin C++ abstraction for platform-specific
scoped handle values.

This is a precursor to moving platform channel C++ types out of the EDK
and into the public library, and ultimately introducing a public C++
invitation API to replace the EDK invitation API.

Bug: 844764
Change-Id: I317decc1deb8cc47eb1000a6a19022465c392d7b
Reviewed-on: https://chromium-review.googlesource.com/1068688Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560866}