Implement overlay scrollbar fade out for non-composited scrollers.

third_party/WebKit/Source/platform/scroll/Scrollbar.h

Index: third_party/WebKit/Source/platform/scroll/Scrollbar.h
diff --git a/third_party/WebKit/Source/platform/scroll/Scrollbar.h b/third_party/WebKit/Source/platform/scroll/Scrollbar.h
index 48b03fa6f58d7ae3600c67367859d3c301273c0a..d8cf6cd5ea87e9d1da4d500ff51c33e508bb517d 100644
--- a/third_party/WebKit/Source/platform/scroll/Scrollbar.h
+++ b/third_party/WebKit/Source/platform/scroll/Scrollbar.h
@@ -105,7 +105,7 @@ class PLATFORM_EXPORT Scrollbar : public Widget,
   ScrollbarPart hoveredPart() const override { return m_hoveredPart; }
 
   void styleChanged() override {}
-  void visibilityChanged() override;
+  void setScrollbarsHidden(bool) override;

[dtapuska, 2016/11/02 16:53:21]

Personally I'd prefer if this was a positive affirmation like
setScrollbarsVisible(bool). But it could just by my mental model.

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

I prefer setScrollbarsHidden for consistency with other plumbing like
Settings::hideScrollbars().  Unless you're going to change it everywhere.

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

I preferred |hidden| to |visible| since visible is the default state and we
really are "hiding" them from hit testing. That said, I don't feel strongly
about it but if we do decide to change it should happen after this patch as
there's lots of plumbing in CC as well.
Huh, didn't even know about that setting. It "hides" scrollbars by not creating
them which is different from the flag I added, perhaps we should rename it to
"NoScrollbars" or something more like that?

   bool enabled() const override { return m_enabled; }
   void setEnabled(bool) override;