Implement currentDOMWindow() instead of currentDocument()

This is a simple, follow-up cleanup after the world refactoring. After this CL, V8Binding.h provides a good set of APIs as follows:

- toDOMWindow
- enteredDOMWindow
- currentDOMWindow
- callingDOMWindow
- toExecutionContext
- currentExecutionContext
- callingExecutionContext

BUG=341032

Review URL: https://codereview.chromium.org/195943010

git-svn-id: svn://svn.chromium.org/blink/trunk@169230 bbb929c8-8fbe-4397-9dbb-9b2b20218538