[iOS] Add ARC Guard to bubble_util.mm

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}

ios/chrome/browser/ui/bubble/bubble_util.mm

@@ -8,6 +8,10 @@
 #include "base/logging.h"
 #import "ios/chrome/browser/ui/bubble/bubble_view.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace bubble_util {
 
 // Calculate the coordinates of the point of the speech bubble's triangle based