Remove WrapperWorldType from DOMDataStore

I'm removing WrapperWorldType from the code base.
What DOMDataStore needs to know is just whether we're in the main world or not.
Thus this CL replaces WrapperWorldType with 'bool m_isMainWorld'.

BUG=341032

Review URL: https://codereview.chromium.org/163913005

git-svn-id: svn://svn.chromium.org/blink/trunk@167338 bbb929c8-8fbe-4397-9dbb-9b2b20218538