HistoryController shouldn't call confusingAndOftenMisusedAttached

It's not entirely clear to me why this check is needed at all.  I tried just
removing it entirely but that caused a test to fail. Given that we just got the
document from a frame, we must be worried about the case where we're in the
middle of unloading the document.

Regardless, checking whether the document has a RenderView is more direct than
asking whether it is "attached".

R=japhet,esprehn
BUG=299011

Review URL: https://chromiumcodereview.appspot.com/24976002

git-svn-id: svn://svn.chromium.org/blink/trunk@158515 bbb929c8-8fbe-4397-9dbb-9b2b20218538