Implement overlay scrollbar fade out for non-composited scrollers.

ui/native_theme/native_theme_aura.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/native_theme/native_theme_aura.h"
 
 #include <limits>
 #include <utility>
 
 #include "base/logging.h"
@@ skipping 179 matching lines @@
   if (state == kDisabled)
     return;
 
   TRACE_EVENT0("blink", "NativeThemeAura::PaintScrollbarThumb");
 
   gfx::Rect thumb_rect(rect);
   SkColor thumb_color;
   SkAlpha thumb_alpha = ThumbAlphaForState(state);
 
   if (IsOverlayScrollbarEnabled()) {
+    if (state == kDisabled)

[skobes, 2016/11/02 21:02:43]

This seems redundant with an identical check at the top of the function.

[bokan, 2016/11/02 22:41:57]

Doh! Missed that :)

+      return;
+
     thumb_color = kOverlayScrollbarThumbColor[theme];
 
     // In overlay mode, draw a stroke (border).
     constexpr int kStrokeWidth = kOverlayScrollbarStrokeWidth;
     SkPaint paint;
     paint.setColor(
         SkColorSetA(kOverlayScrollbarStrokeColor[theme], thumb_alpha));
     paint.setStyle(SkPaint::kStroke_Style);
     paint.setStrokeWidth(kStrokeWidth);
 
@@ skipping 54 matching lines @@
         // branch return 0 for parts that don't exist or assert NOTREACHED.
         // crbug.com/657159.
         break;
     }
   }
 
   return NativeThemeBase::GetPartSize(part, state, extra);
 }
 
 }  // namespace ui