Remove V8Binding::toNativeDOMWindow()

It's confusing that we have both toNativeDOMWindow() and toDOMWindow().
This CL replaces toNativeDOMWindow() with toDOMWindow().

R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/131923003

git-svn-id: svn://svn.chromium.org/blink/trunk@166192 bbb929c8-8fbe-4397-9dbb-9b2b20218538