Re-enable PrerenderVisibility and PrerenderXhrPost tests.

All the failures in build logs are timeouts and the test themselves are no more
interesting than the other basic prerender tests. Re-enable them for now in
hopes that the various prerender changes and test improvements have since
unflaked them.

BUG=249179
TEST=PrerenderBrowserTest.PrerenderVisibility
     PrerenderBrowserTest.PrerenderXhrPost

Review URL: https://codereview.chromium.org/147013003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247014 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247014