[ash-md] Added darkness blending to the colorized shelf.

ash/common/shelf/shelf_constants.h

Index: ash/common/shelf/shelf_constants.h
diff --git a/ash/common/shelf/shelf_constants.h b/ash/common/shelf/shelf_constants.h
index bc783fe942de1e4347db03f521a3e9f6033db9a9..702d5239f214038f88ec7d621e21e081a68ff299 100644
--- a/ash/common/shelf/shelf_constants.h
+++ b/ash/common/shelf/shelf_constants.h
@@ -61,6 +61,13 @@ ASH_EXPORT extern const SkColor kShelfIconColor;
 // The alpha value for the shelf background when a window is overlapping.
 ASH_EXPORT extern const int kShelfTranslucentAlpha;
 
+// The alpha value used to darken a colorized shelf when the shelf is
+// translucent.
+extern const int kShelfTranslucentColorDarkenAlpha;
+
+// The alpha vlaue usesd to darken a colorized shelf when the shelf is opaque.
+extern const int kShelfOpaqueColorDarkenAlpha;
+
 // The width and height of the material design overflow button.
 // TODO(tdanderson): Refactor constants which are common between the shelf
 // and the tray. See crbug.com/623987.