LayerDebugInfo should not clobber trace output

ui/compositor/layer.cc

 // Copyright (c) 2012 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.
 
 #include "ui/compositor/layer.h"
 
 #include <algorithm>
 #include <memory>
 #include <utility>
 
@@ skipping 896 matching lines @@
   return true;
 }
 
 class LayerDebugInfo : public base::trace_event::ConvertableToTraceFormat {
  public:
   explicit LayerDebugInfo(const std::string& name) : name_(name) {}
   ~LayerDebugInfo() override {}
   void AppendAsTraceFormat(std::string* out) const override {
     base::DictionaryValue dictionary;
     dictionary.SetString("layer_name", name_);
-    base::JSONWriter::Write(dictionary, out);
+    std::string tmp;
+    base::JSONWriter::Write(dictionary, &tmp);
+    out->append(tmp);
   }
 
  private:
   std::string name_;
 };
 
 std::unique_ptr<base::trace_event::ConvertableToTraceFormat>
 Layer::TakeDebugInfo(cc::Layer* layer) {
   return base::WrapUnique(new LayerDebugInfo(name_));
 }
@@ skipping 245 matching lines @@
   const auto it = std::find_if(mirrors_.begin(), mirrors_.end(),
       [mirror](const std::unique_ptr<LayerMirror>& mirror_ptr) {
         return mirror_ptr.get() == mirror;
       });
 
   DCHECK(it != mirrors_.end());
   mirrors_.erase(it);
 }
 
 }  // namespace ui