[DevTools] Use device metrics emulation implemented in content.

Source/core/platform/ScrollbarThemeWin.cpp

Index: Source/core/platform/ScrollbarThemeWin.cpp
===================================================================
--- Source/core/platform/ScrollbarThemeWin.cpp	(revision 157211)
+++ Source/core/platform/ScrollbarThemeWin.cpp	(working copy)
@@ -33,6 +33,7 @@
 #include "core/platform/LayoutTestSupport.h"
 #include "core/platform/PlatformMouseEvent.h"
 #include "core/platform/Scrollbar.h"
+#include "core/platform/ScrollbarThemeOverlay.h"
 #include "core/platform/graphics/GraphicsContext.h"
 #include "core/platform/win/SystemInfo.h"
 #include "public/platform/Platform.h"
@@ -42,6 +43,11 @@
 
 ScrollbarTheme* ScrollbarTheme::nativeTheme()
 {
+    if (RuntimeEnabledFeatures::overlayScrollbarsEnabled()) {
+        DEFINE_STATIC_LOCAL(ScrollbarThemeOverlay, theme, (7, 0, ScrollbarThemeOverlay::AllowHitTest));
+        return &theme;
+    }
+
     static ScrollbarThemeWin theme;
     return &theme;
 }