predictors: Add prefetching hit/miss histograms.

chrome/browser/predictors/resource_prefetcher_manager.cc

Index: chrome/browser/predictors/resource_prefetcher_manager.cc
diff --git a/chrome/browser/predictors/resource_prefetcher_manager.cc b/chrome/browser/predictors/resource_prefetcher_manager.cc
index 7130038ed5e8470a3f9c7a435a834c05b81d6806..e703c2118548e7ffca828421d475a0cb26d2bc3e 100644
--- a/chrome/browser/predictors/resource_prefetcher_manager.cc
+++ b/chrome/browser/predictors/resource_prefetcher_manager.cc
@@ -79,14 +79,15 @@ void ResourcePrefetcherManager::MaybeRemovePrefetch(
 }
 
 void ResourcePrefetcherManager::ResourcePrefetcherFinished(
-    ResourcePrefetcher* resource_prefetcher) {
+    ResourcePrefetcher* resource_prefetcher,
+    const ResourcePrefetcher::PrefetcherStats& stats) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
 
   const GURL& main_frame_url = resource_prefetcher->main_frame_url();
   BrowserThread::PostTask(
       BrowserThread::UI, FROM_HERE,
       base::Bind(&ResourcePrefetchPredictor::OnPrefetchingFinished,
-                 base::Unretained(predictor_), main_frame_url));
+                 base::Unretained(predictor_), main_frame_url, stats));

[alexilin, 2017/02/09 15:24:18]

Can we avoid copying of stats object?
I think, we could pass it as unique_ptr from prefetcher to this delegate first
and then to UI thread.

[Benoit L, 2017/02/13 16:13:15]

It was initially because gmock doesn't like unique_ptr, but that's just
laziness. Thank you for not letting that pass.
Done.

 
   const std::string key = main_frame_url.host();
   auto it = prefetcher_map_.find(key);