Implement overlay scrollbar fade out for non-composited scrollers.

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

Index: third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
index 5e7b56694f9eb913b5d9bdfc82b7389b865d9886..fd3b1c14006cff2484246408f5a69c4d539edcbb 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.h
@@ -104,6 +104,8 @@ class PLATFORM_EXPORT ScrollbarTheme {
   int thumbPosition(const ScrollbarThemeClient& scrollbar) {
     return thumbPosition(scrollbar, scrollbar.currentPos());
   }
+  virtual double overlayScrollbarFadeOutDelaySeconds() const;
+  virtual double overlayScrollbarFadeOutDurationSeconds() const;
   // The position the thumb would have, relative to the track, at the specified
   // scroll position.
   virtual int thumbPosition(const ScrollbarThemeClient&, float scrollPosition);