predictors: Add browsertest that tests prefetching.

chrome/browser/predictors/resource_prefetch_predictor.h

Index: chrome/browser/predictors/resource_prefetch_predictor.h
diff --git a/chrome/browser/predictors/resource_prefetch_predictor.h b/chrome/browser/predictors/resource_prefetch_predictor.h
index d4a0065d57f8ec01a19b842eac69fa269d396235..8c48765237ad8f87eb60d73996745748e308b26c 100644
--- a/chrome/browser/predictors/resource_prefetch_predictor.h
+++ b/chrome/browser/predictors/resource_prefetch_predictor.h
@@ -159,6 +159,10 @@ class ResourcePrefetchPredictor
   // |main_frame_url|.
   void StopPrefetching(const GURL& main_frame_url);
 
+  // Called when ResourcePrefetcher is finished, i.e. there is nothing pending
+  // in flight.
+  void OnPrefetchingFinished(const GURL& main_frame_url);
+
   // Sets the |observer| to be notified when the resource prefetch predictor
   // data changes. Previously registered observer will be discarded. Call
   // this with nullptr parameter to de-register observer.
@@ -366,6 +370,8 @@ class TestObserver {
       size_t url_visit_count,
       const ResourcePrefetchPredictor::PageRequestSummary& summary) {}
 
+  virtual void OnPrefetchingFinished(const GURL& main_frame_url) {}
+
   virtual void OnPredictorInitialized() {}
 
  protected: