Revert 241151 "Make RenderFrameHostManager swap RenderFrameHosts..."

r241151 seems to break content_unittests and browser_tests on Chrome OS ASAN bot:
http://build.chromium.org/p/chromium.memory/builders/Linux%20Chromium%20OS%20ASAN%20Tests%20%283%29/builds/12154

> 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

TBR=creis@chromium.org

Review URL: https://codereview.chromium.org/105523006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241159 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241159