Add baked-in favicons for default popular sites on NTP

components/ntp_tiles/icon_cacher.h

 // Copyright 2017 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.
 
 #ifndef COMPONENTS_NTP_TILES_ICON_CACHER_H_
 #define COMPONENTS_NTP_TILES_ICON_CACHER_H_
 
 #include "base/callback.h"
 #include "components/ntp_tiles/popular_sites.h"
 
 namespace ntp_tiles {
 
 // Ensures that a Popular Sites icon is cached, downloading and saving it if
 // not.
 //
 // Does not provide any way to get a fetched favicon; use the FaviconService for
 // that. All this interface guarantees is that FaviconService will be able to
 // get you an icon (if it exists).
 class IconCacher {
  public:
   virtual ~IconCacher() = default;
 
   // Fetches the icon if necessary, then invokes |done| with true if it was
-  // newly fetched (false if it was already cached or could not be fetched).
+  // newly fetched or a default icon was loaded for the first time.
+  // It's invoked with False if it was already cached or could not be fetched.
+  // The callback will be invoked twice if a default icon was returned in the
+  // first place.

[sfiera, 2017/02/27 11:42:20]

This docstring confuses me, probably because it's documenting confusing
behavior. I think the one caller doesn't care about Run(false), so maybe we can
remove those calls and drop the parameter? Then it will be simple: invokes the
callback when a new icon is available (could be twice). I don't remember why the
false notifications existed. Maybe just because it's easier to test; I'm not
sure how best to test if it doesn't call back.

[fhorschig, 2017/02/28 13:13:12]

Dropped the bool. If you don't like this change, I added it in a separate
Patch Set and could also easily split it into a new CL.

MockFunctions (as currently used) support .Times(0) cardinality and a
TestTaskRunner can ensure that the task was finished. As I learned, we
prefer RunLoops over TestTaskRunner, so let me know if you prefer to keep
it like it used to be.

   virtual void StartFetch(PopularSites::Site site,
-                          const base::Callback<void(bool)>& done) = 0;
+                          const base::Callback<void(bool)>& icon_available) = 0;
 };
 
 }  // namespace ntp_tiles
 
 #endif  // COMPONENTS_NTP_TILES_ICON_CACHER_H_