Add function to write paint property tree graph in dot format.

third_party/WebKit/Source/core/paint/PaintPropertyTreePrinter.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef PaintPropertyTreePrinter_h
 #define PaintPropertyTreePrinter_h
 
 #include "core/CoreExport.h"
+#include "wtf/text/WTFString.h"
 
 #ifndef NDEBUG
 
 namespace blink {
 
 class ClipPaintPropertyNode;
 class FrameView;
 class EffectPaintPropertyNode;
 class TransformPaintPropertyNode;
 
 } // namespace blink
 
 // Outside the blink namespace for ease of invocation from gdb.
 CORE_EXPORT void showTransformPropertyTree(const blink::FrameView& rootFrame);
 CORE_EXPORT void showClipPropertyTree(const blink::FrameView& rootFrame);
 CORE_EXPORT void showEffectPropertyTree(const blink::FrameView& rootFrame);
 CORE_EXPORT void showPaintPropertyPath(const blink::TransformPaintPropertyNode*);
 CORE_EXPORT void showPaintPropertyPath(const blink::ClipPaintPropertyNode*);
 CORE_EXPORT void showPaintPropertyPath(const blink::EffectPaintPropertyNode*);
+CORE_EXPORT String paintPropertyTreeGraph(const blink::FrameView&);
 
 #endif // ifndef NDEBUG
 
 #endif // PaintPropertyTreePrinter_h