Remove redundant calls to FrameLoader::frameDetached().

Page::willBeDestroyed() detachs any frames remaining in the frame tree,
so there's no need to do this before calling willBeDestroyed().

In addition, since frameDetached() recursively detaches all frames,
eliminate the loop inside Page::willBeDestroyed() to detach all frames.

Review URL: https://codereview.chromium.org/339493002

git-svn-id: svn://svn.chromium.org/blink/trunk@176173 bbb929c8-8fbe-4397-9dbb-9b2b20218538