predictors: Add browsertest that tests prefetching.

chrome/browser/predictors/resource_prefetch_predictor.cc

Index: chrome/browser/predictors/resource_prefetch_predictor.cc
diff --git a/chrome/browser/predictors/resource_prefetch_predictor.cc b/chrome/browser/predictors/resource_prefetch_predictor.cc
index 76329738d78ca7c27da8d55c8bfa636c3baf9ad5..f901de2382d2b04fe264713f8ccb9217f6a4e957 100644
--- a/chrome/browser/predictors/resource_prefetch_predictor.cc
+++ b/chrome/browser/predictors/resource_prefetch_predictor.cc
@@ -497,6 +497,12 @@ void ResourcePrefetchPredictor::StopPrefetching(const GURL& url) {
                  prefetch_manager_, url));
 }
 
+void ResourcePrefetchPredictor::OnPrefetchingFinished(
+    const GURL& main_frame_url) {
+  if (observer_)
+    observer_->OnPrefetchingFinished(main_frame_url);
+}
+
 void ResourcePrefetchPredictor::SetObserverForTesting(TestObserver* observer) {
   observer_ = observer;
 }