Separate GeoNotifier class from Geolocation

There are some classes in the Geolocation.h/cpp files.
It makes hard to read geolocation implementation, therefore
separate GeoNotifier from Geolocation is better.

BUG=387656

Review URL: https://codereview.chromium.org/393793003

git-svn-id: svn://svn.chromium.org/blink/trunk@178237 bbb929c8-8fbe-4397-9dbb-9b2b20218538