Add UMA histogram data representing the Synced Notifications feature.

1. First, I added a new data point for muting and unmuting the notification
system.

2. Second, I added a new histogram for Synced Notifications that measures
common events that happen with them.

3. In a follow up chekin, I will also measure how often individual
synced notification services are turned off.


BUG=280251

Review URL: https://codereview.chromium.org/26943002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229720 0039d316-1c4b-4281-b951-d872f2087c98
Roll trace-viewer deps from r1036->r1039

TBR=vmpstr

Review URL: https://codereview.chromium.org/29103004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229721 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229720
Update .DEPS.git

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229722 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229721
SVN changes up to revision 229722