Remove DOMWrapperWorld::isIsolatedWorldId and DOMWrapperWorld::context

These methods are unnecessary.

BUG=341032
NOTRY=true

Review URL: https://codereview.chromium.org/168583005

git-svn-id: svn://svn.chromium.org/blink/trunk@167293 bbb929c8-8fbe-4397-9dbb-9b2b20218538