Change multi-prerender API to include per launcher slots.

chrome/renderer/prerender/prerender_dispatcher.cc

Index: chrome/renderer/prerender/prerender_dispatcher.cc
diff --git a/chrome/renderer/prerender/prerender_dispatcher.cc b/chrome/renderer/prerender/prerender_dispatcher.cc
index c85c39e9b4583925d96f8ddda85fd1003af33707..45f54dc3e30a3a252049a186da26da5efacbaf69 100644
--- a/chrome/renderer/prerender/prerender_dispatcher.cc
+++ b/chrome/renderer/prerender/prerender_dispatcher.cc
@@ -59,8 +59,12 @@ void PrerenderDispatcher::OnPrerenderStop(int prerender_id) {
   WebPrerender& prerender = prerenders_[prerender_id];
 
   // The prerender should only be null in unit tests.
-  if (!prerender.isNull())
+  if (!prerender.isNull()) {
+    DVLOG(7) << "stopping...";
     prerender.didStopPrerender();
+  } else {
+    DVLOG(7) << "but it was null. :(";
+  }
 
   // TODO(cbentzel): We'd also want to send the map of active prerenders when
   // creating a new render process, so the Add/Remove go relative to that.