Replaced BackgroundAnimator with ShelfBackgroundAnimator.

ash/common/shelf/overflow_button.cc

Index: ash/common/shelf/overflow_button.cc
diff --git a/ash/common/shelf/overflow_button.cc b/ash/common/shelf/overflow_button.cc
index 6a8585750c7b971d02bd366fab49fbebf2915ea9..03fe492f67312271089524edf48623c9ca9fa567 100644
--- a/ash/common/shelf/overflow_button.cc
+++ b/ash/common/shelf/overflow_button.cc
@@ -31,7 +31,8 @@ OverflowButton::OverflowButton(InkDropButtonListener* listener,
     : CustomButton(nullptr),
       bottom_image_(nullptr),
       listener_(listener),
-      wm_shelf_(wm_shelf) {
+      wm_shelf_(wm_shelf),
+      background_alpha_(0) {
   if (MaterialDesignController::IsShelfMaterial()) {
     bottom_image_md_ =
         CreateVectorIcon(gfx::VectorIconId::SHELF_OVERFLOW, kShelfIconColor);
@@ -51,6 +52,11 @@ void OverflowButton::OnShelfAlignmentChanged() {
   SchedulePaint();
 }
 
+void OverflowButton::SetBackgroundAlpha(int alpha) {
+  background_alpha_ = alpha;
+  SchedulePaint();
+}
+
 void OverflowButton::OnPaint(gfx::Canvas* canvas) {
   gfx::Rect bounds = CalculateButtonBounds();
   PaintBackground(canvas, bounds);
@@ -66,18 +72,9 @@ void OverflowButton::NotifyClick(const ui::Event& event) {
 void OverflowButton::PaintBackground(gfx::Canvas* canvas,
                                      const gfx::Rect& bounds) {
   if (MaterialDesignController::IsShelfMaterial()) {
-    SkColor background_color = SK_ColorTRANSPARENT;
-    if (wm_shelf_->GetBackgroundType() ==
-        ShelfBackgroundType::SHELF_BACKGROUND_DEFAULT) {
-      background_color = SkColorSetA(kShelfBaseColor,
-                                     GetShelfConstant(SHELF_BACKGROUND_ALPHA));
-    }
-
-    // TODO(bruthig|tdanderson): The background should be changed using a
-    // fade in/out animation.
     SkPaint background_paint;
     background_paint.setFlags(SkPaint::kAntiAlias_Flag);
-    background_paint.setColor(background_color);
+    background_paint.setColor(SkColorSetA(kShelfBaseColor, background_alpha_));
     canvas->DrawRoundRect(bounds, kOverflowButtonCornerRadius,
                           background_paint);