Have window.closed return true when frame is closed.

This Window property currently returns true once the Frame it is attached
to is destroyed, which exposes an object memory management detail that's
hard to emulate with Oilpan.

Switch instead to having "closed" return true once the frame is externally
closed or the (i)frame is removed&detached.

Adjust tests to check for this instead -- no indication that this slight
adjustment in behavior will pose a compatibility problem.

R=haraken,ager,mkwst,tkent
BUG=414658

Review URL: https://codereview.chromium.org/538323003

git-svn-id: svn://svn.chromium.org/blink/trunk@182148 bbb929c8-8fbe-4397-9dbb-9b2b20218538