Add testRunner.cancelAllActiveNotifications to cancel all active notifications.

This is to be used from automated GC leak testing to avoid false-positives from documents referenced from un-closed notifications.

BUG=270000

Review URL: https://chromiumcodereview.appspot.com/22852029

git-svn-id: svn://svn.chromium.org/blink/trunk@156684 bbb929c8-8fbe-4397-9dbb-9b2b20218538