Replace PruneAllButActive with PruneAllButVisible.

Before we would forget the entry for the visible page, so we now require there
to be a last committed entry, no transient entry, and optionally a new pending
entry (not an existing pending entry).

BUG=234809
TEST=Prerendering should not swap in without a last committed entry.

Review URL: https://chromiumcodereview.appspot.com/15041004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@203492 0039d316-1c4b-4281-b951-d872f2087c98
12 files changed