Instantiate RenderFrameHosts via a RenderFrameHostFactory indirection.
This relationship is analogous to the RenderViewHost/RenderViewHostFactory
relationship.

Introduce TestRenderFrameHost, a stubbable subclass of RenderFrameHostImpl,
and its factory class, TestRenderFrameHostFactory.

Augment RenderViewTestEnabler so that it installs a TestRenderFrameHostFactory.

BUG=304341
TEST=unittests

Review URL: https://codereview.chromium.org/76063005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236448 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236448