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..b911faa8ab0a1eab2d410f882c6d8037547cbe43 100644
--- a/ui/views/controls/button/label_button_border.cc
+++ b/ui/views/controls/button/label_button_border.cc
@@ -139,14 +139,22 @@ 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 =
+        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);