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It was only needed for ChannelPosix because Mach ports could not be
transferred with the message data. Now that ChannelMac exists, just
TakeHandles() is needed.

Bug: 973882
Change-Id: I83e754ea9877f4d355654ee069b5a6be0a14c1a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1733926Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683747}