Make DOMWrapperWorld::current() return a reference instead of a pointer

Now that it's guaranteed that DOMWrapperWorld is not 0 at any given time, we can make it a reference instead of a pointer.

BUG=341032

Review URL: https://codereview.chromium.org/209713003

git-svn-id: svn://svn.chromium.org/blink/trunk@170261 bbb929c8-8fbe-4397-9dbb-9b2b20218538