Split NativeThemeAura into Overlay and NonOverlay versions.

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 169f96952c08e75c5c6b97b7771f2c7b9ca62cdb..f224f1d303903b9867e1c57711ce61adc0af7420 100644
--- a/content/child/webthemeengine_impl_default.cc
+++ b/content/child/webthemeengine_impl_default.cc
@@ -8,6 +8,7 @@
 #include "skia/ext/platform_canvas.h"
 #include "third_party/WebKit/public/platform/WebRect.h"
 #include "third_party/WebKit/public/platform/WebSize.h"
+#include "third_party/WebKit/public/web/WebRuntimeFeatures.h"
 #include "ui/native_theme/native_theme.h"
 #include "ui/native_theme/overlay_scrollbar_constants_aura.h"
 
@@ -34,6 +35,11 @@ int32_t g_vertical_arrow_bitmap_height;
 int32_t g_horizontal_arrow_bitmap_width;
 #endif
 
+ui::NativeTheme* nativeThemeInstance() {
+  return ui::NativeTheme::GetInstanceForWeb(
+      blink::WebRuntimeFeatures::isOverlayScrollbarsEnabled());
+}
+
 }  // namespace
 
 static ui::NativeTheme::Part NativeThemePart(
@@ -226,7 +232,7 @@ blink::WebSize WebThemeEngineImpl::getSize(WebThemeEngine::Part part) {
       break;
   }
 #endif
-  return ui::NativeTheme::GetInstanceForWeb()->GetPartSize(
+  return nativeThemeInstance()->GetPartSize(
       native_theme_part, ui::NativeTheme::kNormal, extra);
 }
 
@@ -239,7 +245,7 @@ void WebThemeEngineImpl::paint(
   ui::NativeTheme::ExtraParams native_theme_extra_params;
   GetNativeThemeExtraParams(
       part, state, extra_params, &native_theme_extra_params);
-  ui::NativeTheme::GetInstanceForWeb()->Paint(
+  nativeThemeInstance()->Paint(
       canvas, NativeThemePart(part), NativeThemeState(state), gfx::Rect(rect),
       native_theme_extra_params);
 }