Remove call of PrerenderTracker::TryCancelOnIOThread in ChromeContentBrowserClient::CanCreateWindow. I moved cancelling the prerender on the UI thread.

BUG=304341
R=jochen@chromium.org, tburkard@chromium.org

Review URL: https://codereview.chromium.org/112573008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243375 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243375