Check the return value when canceling prerenders.

It's possible for an IO thread cancel to race with the prerender being used on
the UI thread. If the UI thread wins, the prerender was used and we should not
cancel network requests, etc.

TEST=none
BUG=293540

Review URL: https://codereview.chromium.org/23460046

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225322 0039d316-1c4b-4281-b951-d872f2087c98