bindings: Window must be LocalDOMWindow by definition.

Window.idl declares that Window is ImplementedAs=LocalDOMWindow.
It also declares self/window/frames/opener/parent/top attributes
return Window, so LocalDOMWindow::self/window/frames/opener/parent/
top member functions must return LocalDOMWindow, not DOMWindow.

dcheng@, you can change Window.idl to declare ImplementedAs=DOMWindow
instead of LocalDOMWindow, if necessary.  Otherwise, please keep the
consistency here.

BUG=425623

Review URL: https://codereview.chromium.org/700803002

git-svn-id: svn://svn.chromium.org/blink/trunk@184827 bbb929c8-8fbe-4397-9dbb-9b2b20218538