[Refactor] Introduce MediaDelegate

ash/accelerators/accelerator_controller.cc

Index: ash/accelerators/accelerator_controller.cc
diff --git a/ash/accelerators/accelerator_controller.cc b/ash/accelerators/accelerator_controller.cc
index 052baaab3cfbb338f6ad8d115e79a151c8903364..c1eefd93084198d4f8aa919e97a7ecaa3a5bdc93 100644
--- a/ash/accelerators/accelerator_controller.cc
+++ b/ash/accelerators/accelerator_controller.cc
@@ -24,6 +24,7 @@
 #include "ash/launcher/launcher_model.h"
 #include "ash/magnifier/magnification_controller.h"
 #include "ash/magnifier/partial_magnification_controller.h"
+#include "ash/media_delegate.h"
 #include "ash/multi_profile_uma.h"
 #include "ash/new_window_delegate.h"
 #include "ash/root_window_controller.h"
@@ -313,17 +314,17 @@ bool HandleMagnifyScreen(int delta_index) {
 }
 
 bool HandleMediaNextTrack() {
-  Shell::GetInstance()->delegate()->HandleMediaNextTrack();
+  Shell::GetInstance()->media_delegate()->HandleMediaNextTrack();
   return true;
 }
 
 bool HandleMediaPlayPause() {
-  Shell::GetInstance()->delegate()->HandleMediaPlayPause();
+  Shell::GetInstance()->media_delegate()->HandleMediaPlayPause();
   return true;
 }
 
 bool HandleMediaPrevTrack() {
-  Shell::GetInstance()->delegate()->HandleMediaPrevTrack();
+  Shell::GetInstance()->media_delegate()->HandleMediaPrevTrack();
   return true;
 }