Simplify window.closed testing.

Instead of keeping a separate Frame flag, have LocalDOMWindow::closed
consider the presence of a FrameHost on the frame. It'll be cleared
during detach and impending frame closure, and is observationally
equivalent to keeping an unnecessary extra flag.

R=dcheng,ager
BUG=414658

Review URL: https://codereview.chromium.org/544013005

git-svn-id: svn://svn.chromium.org/blink/trunk@182201 bbb929c8-8fbe-4397-9dbb-9b2b20218538