Implement overlay scrollbar fade out for non-composited scrollers.

content/child/webthemeengine_impl_default.cc

Index: content/child/webthemeengine_impl_default.cc
diff --git a/content/child/webthemeengine_impl_default.cc b/content/child/webthemeengine_impl_default.cc
index d55bea603310f74659ac9c358ac27951ff60680a..07ca25b490de932916b8bc4ba53c53951b11b689 100644
--- a/content/child/webthemeengine_impl_default.cc
+++ b/content/child/webthemeengine_impl_default.cc
@@ -9,6 +9,7 @@
 #include "third_party/WebKit/public/platform/WebRect.h"
 #include "third_party/WebKit/public/platform/WebSize.h"
 #include "ui/native_theme/native_theme.h"
+#include "ui/native_theme/overlay_scrollbar_constants_aura.h"
 
 using blink::WebCanvas;
 using blink::WebColor;
@@ -243,6 +244,16 @@ void WebThemeEngineImpl::paint(
       native_theme_extra_params);
 }
 
+double WebThemeEngineImpl::getOverlayScrollbarFadeOutDelaySeconds()
+{
+  return ui::kOverlayScrollbarFadeOutDelay.InSecondsF();
+}
+
+double WebThemeEngineImpl::getOverlayScrollbarFadeOutDurationSeconds()
+{
+  return ui::kOverlayScrollbarFadeOutDuration.InSecondsF();
+}
+
 #if defined(OS_WIN)
 // static
 void WebThemeEngineImpl::cacheScrollBarMetrics(