Move all wallpaper file loading and decoding from DesktopBackgroundController to WallpaperManager.

ash/test/ash_test_base.h

Index: ash/test/ash_test_base.h
diff --git a/ash/test/ash_test_base.h b/ash/test/ash_test_base.h
index 193e71aaf1d24ad78a4594df4f7103a27ef17c66..4ac828e16a1ba25806b3ca74b650861e5fb234a1 100644
--- a/ash/test/ash_test_base.h
+++ b/ash/test/ash_test_base.h
@@ -102,12 +102,10 @@ class AshTestBase : public testing::Test {
     NUMBER_OF_BLOCK_REASONS
   };
 
-  // True if the running environment supports multiple displays,
-  // or false otherwise (e.g. win8 bot).
+  // Proxy to AshTestHelper::SupportsMultipleDisplays().
   static bool SupportsMultipleDisplays();
 
-  // True if the running environment supports host window resize,
-  // or false otherwise (e.g. win8 bot).
+  // Proxy to AshTestHelper::SupportsHostWindowResize().
   static bool SupportsHostWindowResize();
 
   void set_start_session(bool start_session) { start_session_ = start_session; }