Remove calls of PrerenderTracker::TryCancel in ChromeResourceDispatcherHostDelegate::HandleExternalProtocol. This short-circuited cancelling the prerendering on the IO thread. Instead, I moved cancelling the prerender on the UI thread since that calls ends up going to the UI thread right after.

BUG=304341
R=tburkard@chromium.org

Review URL: https://codereview.chromium.org/124393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243095 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243095