Implement a dispatcher for prerender requests

This CL provides a dispatcher that can feed the prerender system with
HTML type requests that were prefetched.

One thing is missing though: this sends all requests in, and doesn't tag the
origin request.  Unfortunately referer information is often tossed out in
resource_dispatcher_host...

Is there a private place it's sent along to for this type of use?  I left that
undone right now but I'm open to suggestions about a good way to do it.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/4655004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@66790 0039d316-1c4b-4281-b951-d872f2087c98
12 files changed