Each SharedMemoryHandle is backed by a HANDLE, and that HANDLE is associated
with a specific process. If a SharedMemoryHandle passed to IPC is associated
with the current process, the IPC stack will automatically broker the handle to
the destination process.

This functionality has been implemented and tested, but is not yet turned on,
because there are a couple of Windows-specific Chrome IPC messages that
intentionally pass a HANDLE associated with another process. I will write a
follow-up CL that turns on this functionality, and removes those IPC messages.

BUG=493414

Review URL: https://codereview.chromium.org/1493413004

Cr-Commit-Position: refs/heads/master@{#368244}