Flags to disable and force devices notifications.

BUG=245379
TBR=noamsml

Review URL: https://codereview.chromium.org/23567049

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224625 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224625