Oilpan: have GeolocationInspectorAgent persistently track its controllers.

With GeolocationController (now back) on the heap, maintain a persistent
set of them from GeolocationInspectorAgent.

R=haraken@chromium.org,ager@chromium.org,zerny@chromium.org
BUG=304341

Review URL: https://codereview.chromium.org/332153002

git-svn-id: svn://svn.chromium.org/blink/trunk@176202 bbb929c8-8fbe-4397-9dbb-9b2b20218538