Oilpan: Rename garbage_collection_type to dom_object_type

For naming consistency, this CL does the following rename.

- garbage_collection_type => gc_type.
- WrappedObjectType => GCType

NOTRY=true

Review URL: https://codereview.chromium.org/229923002

git-svn-id: svn://svn.chromium.org/blink/trunk@171100 bbb929c8-8fbe-4397-9dbb-9b2b20218538