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..2387f462111bc2bf0ed1918521502caf8db4fd4b 100644
--- a/third_party/WebKit/public/platform/WebThemeEngine.h
+++ b/third_party/WebKit/public/platform/WebThemeEngine.h
@@ -163,10 +163,21 @@ class WebThemeEngine {
     int thumbThickness;
     int scrollbarMargin;
     WebColor color;
+    double fadeOutDelaySeconds;
+    double fadeOutDurationSeconds;
   };
 
-  // Gets the overlay scrollbar style. Used for mobile theme.
-  virtual void getOverlayScrollbarStyle(ScrollbarStyle*) {}
+  // Gets the overlay scrollbar style. Not used on Mac.
+  virtual void getOverlayScrollbarStyle(ScrollbarStyle* style) {
+    // Disable overlay scrollbar fade out (for non-composited scrollers) unless
+    // explicitly enabled by the implementing child class. NOTE: these values
+    // aren't used to control Mac fade out - that happens in ScrollAnimatorMac.
+    style->fadeOutDelaySeconds = 0.0;
+    style->fadeOutDurationSeconds = 0.0;
+    // The other fields in this struct are used only on Android to draw solid
+    // color scrollbars. On other platforms the scrollbars are painted in
+    // NativeTheme so these fields are unused in non-Android WebThemeEngines.
+  }
 
   // Paint the given the given theme part.
   virtual void paint(WebCanvas*,