Add page visibility observer for Geolocation

Add page visibility observer for Geolocation so that we stop firing GPS
events to pages that are not visible, in order to save battery life.

R=abarth
BUG=112938

Review URL: https://codereview.chromium.org/27368003

git-svn-id: svn://svn.chromium.org/blink/trunk@159723 bbb929c8-8fbe-4397-9dbb-9b2b20218538