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..169f96952c08e75c5c6b97b7771f2c7b9ca62cdb 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 +244,15 @@ void WebThemeEngineImpl::paint(
       native_theme_extra_params);
 }
 
+void WebThemeEngineImpl::getOverlayScrollbarStyle(ScrollbarStyle* style) {
+  style->fadeOutDelaySeconds = ui::kOverlayScrollbarFadeOutDelay.InSecondsF();
+  style->fadeOutDurationSeconds =
+      ui::kOverlayScrollbarFadeOutDuration.InSecondsF();
+  // 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.
+}
+
 #if defined(OS_WIN)
 // static
 void WebThemeEngineImpl::cacheScrollBarMetrics(