Add interface for telling Drive updates push notification availability.
Even in case we get update notifications from the server,
we poll the updates with less frequency in case updates get lost for some reason.
Use 60 seconds for the polling-only (update notification unavailable) as before, 300 seconds for when update notification is available.
This change applies upon
http://codereview.chromium.org/
11146029/
BUG=150005
TEST=open drive in another browser, make a change and see if the change is reflected immediately (<10sec).
Review URL: https://chromiumcodereview.appspot.com/
11143033
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@163235 0039d316-1c4b-4281-b951-d872f2087c98