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When memory blocks are found by iteration, it is synchronized.  When
the blocks are passed directly by reference, those references need
acquire/release semantics.

Bug: 749328
Change-Id: I87c8888cc2153ab027ee51791fd64d095b1d59e8
Reviewed-on: https://chromium-review.googlesource.com/596273Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491507}