Make some updates to BM bar for MD.

ui/gfx/color_utils.h

Index: ui/gfx/color_utils.h
diff --git a/ui/gfx/color_utils.h b/ui/gfx/color_utils.h
index fd6415eeecc7d437231c3b2de8491c558a882bb4..72dbb58610a4dd66e852c0384ecdb9f6f48e990c 100644
--- a/ui/gfx/color_utils.h
+++ b/ui/gfx/color_utils.h
@@ -20,6 +20,13 @@ struct HSL {
   double l;
 };
 
+// The minimum contrast between text and background that is still readable.
+// This value is taken from w3c accessibility guidelines.
+const double kMinimumReadableContrastRatio = 4.5f;
+
+// Determines the contrast ratio of two colors.
+GFX_EXPORT double GetContrastRatio(SkColor color_a, SkColor color_b);
+
 GFX_EXPORT unsigned char GetLuminanceForColor(SkColor color);
 
 // Calculated according to http://www.w3.org/TR/WCAG20/#relativeluminancedef