Switch "GCM enabled" from tri-state to Boolean
It used to be that GCM could:
1) start automatically
2) start on demand
3) be disabled completely
Options (2) and (3) still exist but (1) is no longer applicable as GCM now
always starts up automatically and shuts down automatically, depending on
whether any apps are registered with it. Thus, the "GCM enabled" state
can be simplified to GCM enabled (starting on demand) / GCM disabled.
BUG=376746
TEST=Manual
Review URL: https://codereview.chromium.org/
291333011
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@273259 0039d316-1c4b-4281-b951-d872f2087c98