Add generic PaletteDelegate code.

ash/common/shell_delegate.h

Index: ash/common/shell_delegate.h
diff --git a/ash/common/shell_delegate.h b/ash/common/shell_delegate.h
index a4c32c446981067880c599b875f11118e6310c24..08551f0e1127a56c03bda3d72956d62f371bb180 100644
--- a/ash/common/shell_delegate.h
+++ b/ash/common/shell_delegate.h
@@ -41,6 +41,7 @@ class AccessibilityDelegate;
 class GPUSupport;
 class MediaDelegate;
 class NewWindowDelegate;
+class PaletteDelegate;
 class PointerWatcherDelegate;
 class SessionStateDelegate;
 class ShelfDelegate;
@@ -123,6 +124,8 @@ class ASH_EXPORT ShellDelegate {
   // Creates a media delegate. Shell takes ownership of the delegate.
   virtual MediaDelegate* CreateMediaDelegate() = 0;
 
+  virtual std::unique_ptr<PaletteDelegate> CreatePaletteDelegate() = 0;
+
   virtual std::unique_ptr<PointerWatcherDelegate>
   CreatePointerWatcherDelegate() = 0;