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 afa66a974aef95d32c6cff09539f77e841e9a2d1..86199958e05acd7017144dc1d7d80f4052799106 100644
--- a/chrome/browser/predictors/resource_prefetch_predictor.cc
+++ b/chrome/browser/predictors/resource_prefetch_predictor.cc
@@ -503,6 +503,12 @@ 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;
 }