Oilpan: Prepare to make GeolocationPosition and GeolocationError garbage-collected.

BUG=340522

Review URL: https://codereview.chromium.org/255423004

git-svn-id: svn://svn.chromium.org/blink/trunk@172447 bbb929c8-8fbe-4397-9dbb-9b2b20218538