Add ability to pass through IDWriteFactory so sandbox can control how it's constructed

public/web/win/WebFontRendering.h

 // Copyright 2014 Google Inc. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef WebFontRendering_h
 #define WebFontRendering_h
 
 #include "public/platform/WebCommon.h"
 
+struct IDWriteFactory;
+
 namespace blink {
 
 class WebFontRendering {
 public:
     BLINK_EXPORT static void setUseDirectWrite(bool);
+    BLINK_EXPORT static void setDirectWriteFactory(IDWriteFactory*);
     BLINK_EXPORT static void setUseSubpixelPositioning(bool);
 };
 
 } // namespace blink
 
 #endif