Cleanup [RuntimeEnabled] use in Window

This removes some cruft added in:
Bindings for CSSOM View smooth scroll API
https://codereview.chromium.org/146003002
...which was done b/c [RuntimeEnabled] didn't work properly on
overloaded methods.

This has been fixed (thanks Jens!) and thus the cruft can be removed.

R=haraken
BUG=339000

Review URL: https://codereview.chromium.org/329303009

git-svn-id: svn://svn.chromium.org/blink/trunk@176088 bbb929c8-8fbe-4397-9dbb-9b2b20218538