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Add ARC guard to bubble_util.mm so it can only be compiled with
Automatic Reference Counting enabled. Although it currently only
contains C++ symbols, it is convention to include the ARC guard on all
new files. If in the future Objective-C symbols were added to the file,
not having the ARC guard could cause memory leaks.

R=edchin@chromium.org, gchatz@chromium.org

Bug: none
Change-Id: I0b72bd93a904f04034ca85f9f21b969f9a358e07
Reviewed-on: https://chromium-review.googlesource.com/596587
Commit-Queue: Cooper Knaak <cooperknaak@google.com>
Reviewed-by: Ed Chin <edchin@chromium.org>
Reviewed-by: Gregory Chatzinoff <gchatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491155}