[ash-md] Wired in the Shelf color to be derived from the Wallpaper.

ash/common/shelf/overflow_button.h

Index: ash/common/shelf/overflow_button.h
diff --git a/ash/common/shelf/overflow_button.h b/ash/common/shelf/overflow_button.h
index 0e1617a4cc80374431a976f632deefc84e2e3ef4..503811de782446b416be8036a0647755f0b7440c 100644
--- a/ash/common/shelf/overflow_button.h
+++ b/ash/common/shelf/overflow_button.h
@@ -24,8 +24,8 @@ class OverflowButton : public views::CustomButton {
   void OnOverflowBubbleShown();
   void OnOverflowBubbleHidden();
 
-  // Sets alpha value of the background and schedules a paint.
-  void SetBackgroundAlpha(int alpha);
+  // Updates background and schedules a paint.
+  void UpdateShelfItemBackground(SkColor color);
 
  private:
   // views::CustomButton:
@@ -66,8 +66,8 @@ class OverflowButton : public views::CustomButton {
   ShelfView* shelf_view_;
   WmShelf* wm_shelf_;
 
-  // Alpha value used to paint the background.
-  int background_alpha_;
+  // Color used to paint the background.
+  SkColor background_color_;
 
   DISALLOW_COPY_AND_ASSIGN(OverflowButton);
 };