Refactor LabelButtonAssetBorder to avoid SkArithmeticMode

ui/views/controls/button/label_button_border.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/views/controls/button/label_button_border.h"
 
 #include "base/logging.h"
 #include "third_party/skia/include/core/SkPaint.h"
 #include "third_party/skia/include/effects/SkArithmeticMode.h"
 #include "ui/base/resource/resource_bundle.h"
@@ skipping 121 matching lines @@
 void LabelButtonAssetBorder::Paint(const View& view, gfx::Canvas* canvas) {
   const NativeThemeDelegate* native_theme_delegate =
       static_cast<const LabelButton*>(&view);
   gfx::Rect rect(native_theme_delegate->GetThemePaintRect());
   ui::NativeTheme::ExtraParams extra;
   const gfx::Animation* animation = native_theme_delegate->GetThemeAnimation();
   ui::NativeTheme::State state = native_theme_delegate->GetThemeState(&extra);
 
   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);
   }
 }
 
 gfx::Size LabelButtonAssetBorder::GetMinimumSize() const {
   gfx::Size minimum_size;
   for (int i = 0; i < 2; ++i) {
     for (int j = 0; j < Button::STATE_COUNT; ++j) {
       if (painters_[i][j])
         minimum_size.SetToMax(painters_[i][j]->GetMinimumSize());
     }
   }
   return minimum_size;
 }
 
 Painter* LabelButtonAssetBorder::GetPainter(bool focused,
                                             Button::ButtonState state) {
   return painters_[focused ? 1 : 0][state].get();
 }
 
 void LabelButtonAssetBorder::SetPainter(bool focused,
                                         Button::ButtonState state,
                                         Painter* painter) {
   painters_[focused ? 1 : 0][state].reset(painter);
 }
 
 }  // namespace views