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 =
+        SkToU8(SkScalarRoundToInt(0xff * animation->GetCurrentValue()));

[sky, 2016/09/29 16:05:17]

CurrentValueBetween(0, 255)?

[f(malita), 2016/09/29 16:35:28]

Done.

+
     const SkRect sk_rect = gfx::RectToSkRect(rect);
-    canvas->sk_canvas()->saveLayer(&sk_rect, NULL);
+    canvas->sk_canvas()->saveLayer(&sk_rect, nullptr);
+
+    // First, modulate the background by 1 - alpha.
+    canvas->sk_canvas()->saveLayerAlpha(&sk_rect, 0xff - fg_alpha);
     state = native_theme_delegate->GetBackgroundThemeState(&extra);
     PaintHelper(this, canvas, state, rect, extra);
+    canvas->sk_canvas()->restore();

[sky, 2016/09/29 16:05:17]

All these raw save/restore make me nervous as they are easy to get wrong. Are
there any objects that do the savelayer in their constructor and restore in the
destructor?

[f(malita), 2016/09/29 16:35:28]

We have SkAutoCanvasRestore, which restores to the initial save count in dtor,
and optionally performs a save() in ctor.  It doesn't know about
saveLayer()/saveLayerAlpha(), but we could still use it for restoring
automatically when exiting the scope.

Let me know if patch set #4 looks clearer to you, or we should stick to explicit
restore.

 
+    // 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);
   }
 }
@@ skipping 14 matching lines @@
   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