Refactor LabelButtonAssetBorder to avoid SkArithmeticMode

ui/views/controls/button/label_button_border.cc

Index: ui/views/controls/button/label_button_border.cc
diff --git a/ui/views/controls/button/label_button_border.cc b/ui/views/controls/button/label_button_border.cc
index e8347bb9c7938f539cc00f0222c1b1b0885eac04..6c3f3ea7c84076038ec248dc822d0249e0e2710d 100644
--- a/ui/views/controls/button/label_button_border.cc
+++ b/ui/views/controls/button/label_button_border.cc
@@ -139,20 +139,28 @@ void LabelButtonAssetBorder::Paint(const View& view, gfx::Canvas* canvas) {
 
   if (animation && animation->is_animating()) {
     // Linearly interpolate background and foreground painters during animation.
+    uint8_t fg_alpha =
+        static_cast<uint8_t>(animation->CurrentValueBetween(0, 255));
+
     const SkRect sk_rect = gfx::RectToSkRect(rect);
-    canvas->sk_canvas()->saveLayer(&sk_rect, NULL);
-    state = native_theme_delegate->GetBackgroundThemeState(&extra);
-    PaintHelper(this, canvas, state, rect, extra);
+    SkAutoCanvasRestore auto_restore(canvas->sk_canvas(), false);
+    canvas->sk_canvas()->saveLayer(&sk_rect, nullptr);
+
+    {
+      // First, modulate the background by 1 - alpha.
+      SkAutoCanvasRestore auto_restore(canvas->sk_canvas(), false);
+      canvas->sk_canvas()->saveLayerAlpha(&sk_rect, 255 - fg_alpha);
+      state = native_theme_delegate->GetBackgroundThemeState(&extra);
+      PaintHelper(this, canvas, state, rect, extra);
+    }
 
+    // Then modulate the foreground by alpha, and blend using kPlus_Mode.
     SkPaint paint;
-    double scale = animation->GetCurrentValue();
-    paint.setXfermode(SkArithmeticMode::Make(0.0f, scale, 1.0 - scale, 0.0));
+    paint.setAlpha(fg_alpha);
+    paint.setXfermodeMode(SkXfermode::kPlus_Mode);
     canvas->sk_canvas()->saveLayer(&sk_rect, &paint);
     state = native_theme_delegate->GetForegroundThemeState(&extra);
     PaintHelper(this, canvas, state, rect, extra);
-    canvas->sk_canvas()->restore();
-
-    canvas->sk_canvas()->restore();
   } else {
     PaintHelper(this, canvas, state, rect, extra);
   }