Prevent a null crash when window.history is called during detach.

In certain rare cases, history.{push,replace}State are called when during
plugin detach. In this case, it's possible for the state modification to be
requested on a frame that is half-way detached. The Document is still live,
but the DocumentLoader is gone. Exit early in this case.

BUG=356424

Review URL: https://codereview.chromium.org/215183002

git-svn-id: svn://svn.chromium.org/blink/trunk@170219 bbb929c8-8fbe-4397-9dbb-9b2b20218538