ash/chromeos: Add shift-alt-p shortcut to open palette.

ash/common/palette_delegate.h

Index: ash/common/palette_delegate.h
diff --git a/ash/common/palette_delegate.h b/ash/common/palette_delegate.h
index 6daf59e8e4ea2c16a88a8a8a3774f1647f335098..c84bc00cb1226828694968214e63020337d39121 100644
--- a/ash/common/palette_delegate.h
+++ b/ash/common/palette_delegate.h
@@ -47,6 +47,10 @@ class PaletteDelegate {
   // event.
   virtual bool ShouldAutoOpenPalette() = 0;
 
+  // Returns true if the palette should be displayed. This is the one-shot
+  // equivalent to AddPaletteEnableListener.
+  virtual bool ShouldShowPalette() = 0;
+
   // Take a screenshot of the entire window.
   virtual void TakeScreenshot() = 0;