CustomButton cleanup: make protected members private, create accessors

ash/shelf/overflow_button.cc

Index: ash/shelf/overflow_button.cc
diff --git a/ash/shelf/overflow_button.cc b/ash/shelf/overflow_button.cc
index 79b2bcdfa040953c4276430ba38325b0978660a6..66bc2b8fc0a7e58b923b709d48bef3b90fac2309 100644
--- a/ash/shelf/overflow_button.cc
+++ b/ash/shelf/overflow_button.cc
@@ -71,9 +71,9 @@ void OverflowButton::PaintBackground(gfx::Canvas* canvas, int alpha) {
   SkPaint paint;
   paint.setAntiAlias(true);
   paint.setStyle(SkPaint::kFill_Style);
-  paint.setColor(SkColorSetARGB(
-      kButtonHoverAlpha * hover_animation_->GetCurrentValue(),
-      0, 0, 0));
+  paint.setColor(
+      SkColorSetA(SK_ColorBLACK,
+                  hover_animation().CurrentValueBetween(0, kButtonHoverAlpha)));
 
   const SkScalar radius = SkIntToScalar(kButtonCornerRadius);
   SkPath path;