exo: Add "back" button to shell surface decorations.

ash/frame/custom_frame_view_ash.h

Index: ash/frame/custom_frame_view_ash.h
diff --git a/ash/frame/custom_frame_view_ash.h b/ash/frame/custom_frame_view_ash.h
index de63451c732dffd5086281d4ed134d1f2845634b..652fdde5337768086f92f7e648c1fa0513beaa77 100644
--- a/ash/frame/custom_frame_view_ash.h
+++ b/ash/frame/custom_frame_view_ash.h
@@ -12,6 +12,7 @@
 
 namespace ash {
 class FrameBorderHitTestController;
+class FrameCaptionButton;
 class FrameCaptionButtonContainerView;
 class ImmersiveFullscreenController;
 }
@@ -46,6 +47,9 @@ class ASH_EXPORT CustomFrameViewAsh : public views::NonClientFrameView {
   // will have some transparency added when the frame is drawn.
   void SetFrameColors(SkColor active_frame_color, SkColor inactive_frame_color);
 
+  // Sets the left caption button.
+  void SetLeftCaptionButton(FrameCaptionButton* left_caption_button);
+
   // views::NonClientFrameView:
   gfx::Rect GetBoundsForClientView() const override;
   gfx::Rect GetWindowBoundsForClientBounds(