[AiS] Add prefetching for Answers images.

chrome/browser/bitmap_fetcher/bitmap_fetcher_service.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/bitmap_fetcher/bitmap_fetcher_service.h"
 
 #include "base/memory/weak_ptr.h"
 #include "chrome/browser/bitmap_fetcher/bitmap_fetcher.h"
 #include "chrome/browser/profiles/profile.h"
 #include "net/base/load_flags.h"
@@ skipping 96 matching lines @@
 
   // Make sure there's a fetcher for this URL and attach to request.
   const chrome::BitmapFetcher* fetcher = EnsureFetcherForUrl(url);
   request->set_fetcher(fetcher);
 
   requests_.push_back(request.release());
   return requests_.back()->request_id();
 }
 
 void BitmapFetcherService::Prefetch(const GURL& url) {
-  EnsureFetcherForUrl(url);
+  if (url.is_valid())

[sky, 2014/06/20 15:13:56]

If you're going to check the url here, shouldn't you also check it in
RequestImage?

+    EnsureFetcherForUrl(url);
 }
 
 chrome::BitmapFetcher* BitmapFetcherService::CreateFetcher(const GURL& url) {
   chrome::BitmapFetcher* new_fetcher = new chrome::BitmapFetcher(url, this);
 
   new_fetcher->Start(
       context_->GetRequestContext(),
       std::string(),
       net::URLRequest::CLEAR_REFERRER_ON_TRANSITION_FROM_SECURE_TO_INSECURE,
       net::LOAD_NORMAL);
@@ skipping 53 matching lines @@
   }
 
   if (!bitmap->isNull()) {
     CacheEntry* entry = new CacheEntry;
     entry->bitmap.reset(new SkBitmap(*bitmap));
     cache_.Put(fetcher->url(), entry);
   }
 
   RemoveFetcher(fetcher);
 }