Disabling flaky PrerenderBrowserTest.PrerenderPageNewTab.

TBR=estade@chromium.org
BUG=335835

Review URL: https://codereview.chromium.org/137533017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245710 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245710