Remove last remnants of back/forward cache

The DocumentWillBecomeInactive ReasonForSuspension is no longer used. This
allows Prerenderer to convert from ActiveDOMObject to just a
DocumentLifecycleObserver. HTMLMediaElement still needs to be an
ActiveDOMObject for hasPendingActivity.

In addition, clean up two manual calls to stopActiveDOMObjects in
FrameLoader.cpp. They predate the call in Document::detach and, in both cases,
Document::detach would shortly be called if it had not been already.

BUG=none

Review URL: https://codereview.chromium.org/26613008

git-svn-id: svn://svn.chromium.org/blink/trunk@159309 bbb929c8-8fbe-4397-9dbb-9b2b20218538