Implement overlay scrollbar fade out for non-composited scrollers.

third_party/WebKit/public/platform/WebThemeEngine.h

Index: third_party/WebKit/public/platform/WebThemeEngine.h
diff --git a/third_party/WebKit/public/platform/WebThemeEngine.h b/third_party/WebKit/public/platform/WebThemeEngine.h
index 8fe876d11d9b8f6647bde8d53955354c46014e4d..16b94f3b5c61fa4064e9ede9b3a88c88e2e7c19a 100644
--- a/third_party/WebKit/public/platform/WebThemeEngine.h
+++ b/third_party/WebKit/public/platform/WebThemeEngine.h
@@ -165,6 +165,9 @@ class WebThemeEngine {
     WebColor color;
   };
 
+  virtual double getOverlayScrollbarFadeOutDelaySeconds() { return 0.0; }

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

Could these just be fields in ScrollbarStyle and reuse getOverlayScrollbarStyle?

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

Yup, done. I also added a TODO to set those members for Android so we can get
the same behavior for non-composited scrollers in Android. That'll require some
plumbing to move the constants into a single place so I'll leave that for a
future patch.

+  virtual double getOverlayScrollbarFadeOutDurationSeconds() { return 0.0; }
+
   // Gets the overlay scrollbar style. Used for mobile theme.
   virtual void getOverlayScrollbarStyle(ScrollbarStyle*) {}