Don't request missing favicon on every page request.

chrome/browser/favicon/favicon_service.cc

 // Copyright (c) 2012 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/favicon/favicon_service.h"
 
+#include "base/hash.h"
 #include "base/message_loop_proxy.h"
 #include "chrome/browser/favicon/favicon_util.h"
 #include "chrome/browser/history/history_backend.h"
 #include "chrome/browser/history/history_service.h"
 #include "chrome/browser/history/history_service_factory.h"
 #include "chrome/browser/history/select_favicon_frames.h"
 #include "chrome/browser/ui/webui/chrome_web_ui_controller_factory.h"
 #include "chrome/common/url_constants.h"
 #include "extensions/common/constants.h"
 #include "third_party/skia/include/core/SkBitmap.h"
@@ skipping 329 matching lines @@
   if (!gfx::PNGCodec::EncodeBGRASkBitmap(resized_image.AsBitmap(), false,
                                          &resized_bitmap_data)) {
     callback.Run(history::FaviconBitmapResult());
     return;
   }
 
   bitmap_result.bitmap_data = base::RefCountedBytes::TakeVector(
       &resized_bitmap_data);
   callback.Run(bitmap_result);
 }
+
+void FaviconService::AddMissingFavicon(const GURL& icon_url) {
+  missing_favicon_urls_.insert(base::Hash(icon_url.spec()));

[mef, 2013/04/29 21:55:03]

Should I somehow protect this set from multi-threaded access?

[pauljensen, 2013/04/30 14:38:04]

I would always tend to say yes, but I don't see any thread checks in this file. 
If all calls are on the same thread and it looks like this will always be the
case, then maybe not.  I'm not knowledgeable enough when it comes to
FaviconService to give a good answer, an OWNER of this file would know best.

[mef, 2013/04/30 21:57:03]

Sounds good, will do.
On 2013/04/30 14:38:04, pauljensen wrote:

+}
+
+bool FaviconService::IsMissingFavicon(const GURL& icon_url) const {
+  int32 url_hash = base::Hash(icon_url.spec());
+  return missing_favicon_urls_.find(url_hash) != missing_favicon_urls_.end();
+}
+