Add generic PaletteDelegate code.

ash/test/test_shell_delegate.cc

Index: ash/test/test_shell_delegate.cc
diff --git a/ash/test/test_shell_delegate.cc b/ash/test/test_shell_delegate.cc
index 64e87b473672fab475e7eeadb5cfe440318a422b..28412c93a7011d7b4fa7bf9df6a24959db75c1a7 100644
--- a/ash/test/test_shell_delegate.cc
+++ b/ash/test/test_shell_delegate.cc
@@ -12,6 +12,7 @@
 #include "ash/common/gpu_support_stub.h"
 #include "ash/common/media_delegate.h"
 #include "ash/common/new_window_delegate.h"
+#include "ash/common/palette_delegate.h"
 #include "ash/common/session/session_state_delegate.h"
 #include "ash/common/shell_window_ids.h"
 #include "ash/common/wm/window_state.h"
@@ -186,6 +187,10 @@ MediaDelegate* TestShellDelegate::CreateMediaDelegate() {
   return new MediaDelegateImpl;
 }
 
+std::unique_ptr<PaletteDelegate> TestShellDelegate::CreatePaletteDelegate() {
+  return nullptr;
+}
+
 std::unique_ptr<PointerWatcherDelegate>
 TestShellDelegate::CreatePointerWatcherDelegate() {
   return base::WrapUnique(new PointerWatcherDelegateAura);