Make DOMWrapperWorld::current() callable from workers

This CL does the following two things:

- Makes DOMWrapperWorld::current() callable from workers. Now that a world is always stored in PerContextDataHolder::world(), DOMWrapperWorld::current() can just look up the PerContextDataHolder::world().

- Makes DOMWrapperWorld::current() callable even when a window object is being initialized. It's possible that some methods call DOMWrapperWorld::current() while the window is being initialized. To handles the subtle situation, this CL adds DOMWrapperWorld::worldOfInitializingWindow.


BUG=341032

Review URL: https://codereview.chromium.org/182903003

git-svn-id: svn://svn.chromium.org/blink/trunk@168898 bbb929c8-8fbe-4397-9dbb-9b2b20218538