Remove unnecessary NULL check in DOMWindow::top()

https://chromium.googlesource.com/chromium/blink/+/c07c40ae84d58b96cb44ad6c03d89a1b5438470b

The use of |page| in DOMWindow::top() was replaced by going to the
frame tree, but the NULL check for |page| was not removed. Remove it.

BUG=none

Review URL: https://codereview.chromium.org/289983004

git-svn-id: svn://svn.chromium.org/blink/trunk@174148 bbb929c8-8fbe-4397-9dbb-9b2b20218538