Add the supporting code for the cast system tray integration

ash/system/tray/system_tray_delegate.h

Index: ash/system/tray/system_tray_delegate.h
diff --git a/ash/system/tray/system_tray_delegate.h b/ash/system/tray/system_tray_delegate.h
index fb2601af93c239dfbfca0cb7db759b4dcf45c6d6..857ee09359497ebe0b25abc5558b9eaf02b4d84b 100644
--- a/ash/system/tray/system_tray_delegate.h
+++ b/ash/system/tray/system_tray_delegate.h
@@ -98,6 +98,7 @@ struct ASH_EXPORT UpdateInfo {
 
 using IMEInfoList = std::vector<IMEInfo>;
 
+class CastConfigDelegate;
 class NetworkingConfigDelegate;
 class VPNDelegate;
 
@@ -284,6 +285,9 @@ class ASH_EXPORT SystemTrayDelegate {
   // Shows UI for changing proxy settings.
   virtual void ChangeProxySettings();
 
+  // Returns CastConfigDelegate. May return nullptr.
+  virtual CastConfigDelegate* GetCastConfigDelegate() const;
+
   // Returns NetworkingConfigDelegate. May return nullptr.
   virtual NetworkingConfigDelegate* GetNetworkingConfigDelegate() const;