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An errant or malicious process could send Mach messages with extra
descriptors. While the message wouldn't be processed, it would cause the
rights to leak in the browser.

When handling a request, add additional checks for the msgh_id and
msgh_size. If the message does not pass muster, or it is from a process
that is not expected, ensure any rights contained within are destroyed.

Change-Id: If333372c72cb20d99713c78d5451c04a3321113f
Reviewed-on: https://chromium-review.googlesource.com/c/1305394
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603639}