Add MetalayerMode to the palette.

ash/palette_delegate.h

Index: ash/palette_delegate.h
diff --git a/ash/palette_delegate.h b/ash/palette_delegate.h
index 942f09e7cc541a535702374c07a60e68506e86c4..8f370d6ba80fbaa097065a4113645937c82fd053 100644
--- a/ash/palette_delegate.h
+++ b/ash/palette_delegate.h
@@ -54,6 +54,17 @@ class PaletteDelegate {
   // Cancels any active partial screenshot session.
   virtual void CancelPartialScreenshot() = 0;
 
+  // Returns true if the metalayer is available.
+  virtual bool IsMetalayerSupported() = 0;
+
+  // Shows the metalayer.
+  // |closed| will be invoked when the metalayer is closed or
+  // if the metalayer could not be shown.
+  virtual void ShowMetalayer(const base::Closure& closed) = 0;
+
+  // Hides the metalayer.
+  virtual void HideMetalayer() = 0;
+
  private:
   DISALLOW_ASSIGN(PaletteDelegate);
 };