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In order to facilitate the move to MessageCenterNotificationManager, we
need to be able to determine when message_center.gyp is being included
in the build.  This change defines ENABLE_MESSAGE_CENTER which can be
used for conditional compilation.

r=stevenjb@chromium.org
TBR=ben@chromium.org

BUG=168605

Review URL: https://chromiumcodereview.appspot.com/11791004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@175876 0039d316-1c4b-4281-b951-d872f2087c98