Don't change PrerenderTracker state in WillTransferRequestToNewProcess

Not all of the prerender state was being updated in that function. Instead,
DCHECK that we're not in a prerender. Prerender can't handle cross-process
navigations right now and should abort the prerender before the navigation
occurs. This reverts the PrerenderTracker parts of
https://src.chromium.org/viewvc/chrome?revision=225263&view=revision

Add a browser test which tests that prerenders are aborted correctly. Also
clean up a bit of %-escaping for redirects in the tests.

BUG=298988,106407
TEST=PrerenderBrowserTest.PrerenderCrossProcessServerRedirect

Review URL: https://codereview.chromium.org/24810003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225987 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225987