Add the preference message_center.welcome_notification_dismissed

To ensure users only get one welcome notification, notifications will use a
synced preference to track if a welcome notification was shown to the user.

Once the synced notification is marked as shown, all Chrome instances
can hide the welcome notification

BUG=305449

Review URL: https://codereview.chromium.org/26865005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228275 0039d316-1c4b-4281-b951-d872f2087c98
Introduce SKIP_TEST_IF_MEDIA_CODEC_BRIDGE_IS_NOT_AVAILABLE.

We use to skip tests when MediaCodecBridge is not available.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/27029005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228276 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228276
Blink roll 159427:159462

http://build.chromium.org/f/chromium/perf/dashboard/ui/changelog_blink.html?url=/trunk&range=159428:159462&mode=html
TBR=
BUG=

Review URL: https://codereview.chromium.org/26905006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228278 0039d316-1c4b-4281-b951-d872f2087c98
Update .DEPS.git

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228279 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228278
SVN changes up to revision 228279
Cleans up some of the comments in TabDragController that are stale

BUG=none
TEST=none
R=ben@chromium.org
TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/26799008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228280 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228280