Don't cancel prerenders right after abandoning them.

When a Document is garbage-collected on navigate away, the LinkLoader notices
and calls cancel. But the Prerender may already have been abandoned from a
navigation by the Prerenderer (which shares ownership of the Prerender with the
LinkLoader). Let the first of these take precedence.

BUG=83901

Review URL: https://chromiumcodereview.appspot.com/23591032

git-svn-id: svn://svn.chromium.org/blink/trunk@157831 bbb929c8-8fbe-4397-9dbb-9b2b20218538