Use "Dictionary" for PositionOptions instead of Custom binding

getCurrentPosition and watchPosition of Geolocation API use "Custom"
binding for implementing PositionOptions.
But it could be implemented using "Dictionary" instead of "Custom"
and we can remove custom binding file(V8GeolocationCustom.cpp) for that.

BUG=385426

Review URL: https://codereview.chromium.org/336343002

git-svn-id: svn://svn.chromium.org/blink/trunk@176719 bbb929c8-8fbe-4397-9dbb-9b2b20218538