Add SystemTrayObservers

ash/system/tray/test_system_tray_delegate.cc

Index: ash/system/tray/test_system_tray_delegate.cc
diff --git a/ash/system/tray/test_system_tray_delegate.cc b/ash/system/tray/test_system_tray_delegate.cc
index 7602c4f64f0ca0741c92169e9b221e6dc2deda59..f6d385b69c228cc24f15516fb2c70f1486d21838 100644
--- a/ash/system/tray/test_system_tray_delegate.cc
+++ b/ash/system/tray/test_system_tray_delegate.cc
@@ -62,6 +62,10 @@ TestSystemTrayDelegate::~TestSystemTrayDelegate() {
 void TestSystemTrayDelegate::Initialize() {
 }
 
+SystemTrayObservers* TestSystemTrayDelegate::GetSystemTrayObservers() {
+  return NULL;
+}
+
 bool TestSystemTrayDelegate::GetTrayVisibilityOnStartup() {
   return true;
 }