Make RenderFrameHostManager swap RenderFrameHosts, not RenderViewHosts.

We still only have a RFHM for the main frame and not subframes, unless
the --site-per-process flag is passed.  To external callers, RFHM is
still effectively swapping RenderViewHosts.

RenderFrameHosts now indirectly keep their RenderViewHosts alive.

BUG=314791
TEST=No visible behavior change.

Review URL: https://codereview.chromium.org/106963004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241151 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241151