Use dark scrollbar theme for transparent frames.

third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  * Copyright (C) 2008, 2011 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 418 matching lines @@
     Color background_color) {
   ScrollbarOverlayColorTheme old_overlay_theme =
       GetScrollbarOverlayColorTheme();
   ScrollbarOverlayColorTheme overlay_theme = kScrollbarOverlayColorThemeDark;
 
   // Reduce the background color from RGB to a lightness value
   // and determine which scrollbar style to use based on a lightness
   // heuristic.
   double hue, saturation, lightness;
   background_color.GetHSL(hue, saturation, lightness);
-  if (lightness <= .5)
+  if (lightness <= .5 && background_color.Alpha())

[jbroman, 2017/04/26 18:10:20]

Possibly the ideal threshold is a little higher than 1/255, but I'm fine with
whatever.

[bokan, 2017/04/26 18:11:58]

Yeah, I thought about that but the scrollbar theme is already mostly a wild
guess so this is just a fix to a common occurrence. We'll adjust if it comes up.

     overlay_theme = kScrollbarOverlayColorThemeLight;
 
   if (old_overlay_theme != overlay_theme)
     SetScrollbarOverlayColorTheme(overlay_theme);
 }
 
 void ScrollableArea::SetScrollbarNeedsPaintInvalidation(
     ScrollbarOrientation orientation) {
   if (orientation == kHorizontalScrollbar) {
     if (GraphicsLayer* graphics_layer = LayerForHorizontalScrollbar()) {
@@ skipping 221 matching lines @@
                                          offset.y() - ScrollOrigin().Y());
   SetScrollOffset(new_offset, kCompositorScroll);
 }
 
 DEFINE_TRACE(ScrollableArea) {
   visitor->Trace(scroll_animator_);
   visitor->Trace(programmatic_scroll_animator_);
 }
 
 }  // namespace blink