Merge PrerenderHandle into Prerender

PrerenderHandle is a reference-counted wrapper over a RefPtr<Prerender>.
Convert references to PrerenderHandle to a reference to Prerender itself.

Also add some missing calls to removeClient in LinkLoader. The removeClient
call in PrerenderHandle's destructor is lost now, and those calls should have
been in LinkLoader anyway since the LinkLoader is the client.

BUG=297708

Review URL: https://chromiumcodereview.appspot.com/23600073

git-svn-id: svn://svn.chromium.org/blink/trunk@158382 bbb929c8-8fbe-4397-9dbb-9b2b20218538