Add baked-in favicons for default popular sites on NTP

components/ntp_tiles/icon_cacher.h

Index: components/ntp_tiles/icon_cacher.h
diff --git a/components/ntp_tiles/icon_cacher.h b/components/ntp_tiles/icon_cacher.h
index 22beeb6f48c415b3616e15c8c1a94a6b4b9b3c23..d3aff0651c523ecddd93ce0b334d558014cb607b 100644
--- a/components/ntp_tiles/icon_cacher.h
+++ b/components/ntp_tiles/icon_cacher.h
@@ -21,9 +21,12 @@ class IconCacher {
   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