Make panel window's header color customizable in Ash.

ash/wm/panels/panel_frame_view.h

Index: ash/wm/panels/panel_frame_view.h
diff --git a/ash/wm/panels/panel_frame_view.h b/ash/wm/panels/panel_frame_view.h
index f4ba33be8475ac57aafa7dadb1f9e1473a57254e..d51a72ae27e74d337b346d9b94f5ecb0abc15800 100644
--- a/ash/wm/panels/panel_frame_view.h
+++ b/ash/wm/panels/panel_frame_view.h
@@ -31,6 +31,10 @@ class ASH_EXPORT PanelFrameView : public views::NonClientFrameView {
   PanelFrameView(views::Widget* frame, FrameType frame_type);
   ~PanelFrameView() override;
 
+  // Sets the active and inactive frame colors. Note the inactive frame color
+  // will have some transparency added when the frame is drawn.
+  void SetFrameColors(SkColor active_frame_color, SkColor inactive_frame_color);
+
   // Overridden from views::View:
   const char* GetClassName() const override;