predictors: Add browsertest that tests prefetching.

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 476ba79832045499ae21116bf822da2542bb2be5..7130038ed5e8470a3f9c7a435a834c05b81d6806 100644
--- a/chrome/browser/predictors/resource_prefetcher_manager.cc
+++ b/chrome/browser/predictors/resource_prefetcher_manager.cc
@@ -82,7 +82,13 @@ void ResourcePrefetcherManager::ResourcePrefetcherFinished(
     ResourcePrefetcher* resource_prefetcher) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
 
-  const std::string key = resource_prefetcher->main_frame_url().host();
+  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));
+
+  const std::string key = main_frame_url.host();
   auto it = prefetcher_map_.find(key);
   DCHECK(it != prefetcher_map_.end());
   prefetcher_map_.erase(it);