Fix crash in GeolocationController when the page gets destroyed

Fix crash in GeolocationController when the page gets destroyed by
checking that page() does not return null before using it.

BUG=310326
TEST=fast/dom/Geolocation/window-close-crash.html

Review URL: https://codereview.chromium.org/30363003

git-svn-id: svn://svn.chromium.org/blink/trunk@160351 bbb929c8-8fbe-4397-9dbb-9b2b20218538
This patch resets "leave this page" decision in the
BeforeUnload confirmation dialog whenever
the user cancels the whole process of closing the browser
window. 

The corresponding tests on the chromium-side will be
disabled at first:
https://codereview.chromium.org/37163002/

... and then fixed after the blink-roll:
https://codereview.chromium.org/32693002/

BUG=286301

Review URL: https://codereview.chromium.org/31093002

git-svn-id: svn://svn.chromium.org/blink/trunk@160352 bbb929c8-8fbe-4397-9dbb-9b2b20218538