Oilpan: detach GeolocationController on being destroyed as a supplement.

If the LocalFrame supplementable signals destruction, detach from the
inspector agent and clear out the GeolocationClient reference after
having relayed the destruction notification.

Not doing both risks the inspector agent object accessing a what's
now effectively a dead object (with a dead client reference; for safety
we clear that out also.)

R=haraken@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/337053006

git-svn-id: svn://svn.chromium.org/blink/trunk@176692 bbb929c8-8fbe-4397-9dbb-9b2b20218538