Implement overlay scrollbar fade out for non-composited scrollers.

content/child/webthemeengine_impl_default.h

Index: content/child/webthemeengine_impl_default.h
diff --git a/content/child/webthemeengine_impl_default.h b/content/child/webthemeengine_impl_default.h
index cf1eda10508142f83d1ebdc179b0480df5eede72..c1d8093b340a2184835ec7965d1f306b5e527353 100644
--- a/content/child/webthemeengine_impl_default.h
+++ b/content/child/webthemeengine_impl_default.h
@@ -21,6 +21,8 @@ class WebThemeEngineImpl : public blink::WebThemeEngine {
              blink::WebThemeEngine::State state,
              const blink::WebRect& rect,
              const blink::WebThemeEngine::ExtraParams* extra_params) override;
+  double getOverlayScrollbarFadeOutDelaySeconds() override;
+  double getOverlayScrollbarFadeOutDurationSeconds() override;
 #if defined(OS_WIN)
   // Caches the scrollbar metrics. These are retrieved in the browser and passed
   // to the renderer in RendererPreferences because the required Windows system