Change DataPersistent constructor to allow one line initialisation.

third_party/WebKit/Source/core/style/DataPersistent.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 DataPersistent_h
 #define DataPersistent_h
 
 #include <memory>
 #include "platform/heap/Handle.h"
 #include "platform/wtf/Allocator.h"
@@ skipping 11 matching lines @@
 // on a copy of the underlying T object.
 //
 // DataPersistent<T> does assume that no one keeps non-DataPersistent<> shared
 // references to the underlying T object that it manages, and is mutating the
 // object via those.
 template <typename T>
 class DataPersistent {
   USING_FAST_MALLOC(DataPersistent);
 
  public:
-  DataPersistent() : own_copy_(false) {}
+  DataPersistent()
+      : data_(WTF::WrapUnique(new Persistent<T>(T::Create()))),
+        own_copy_(true) {}
+
+  DataPersistent(std::nullptr_t) : data_(nullptr), own_copy_(false) {}
 
   DataPersistent(const DataPersistent& other) : own_copy_(false) {
     if (other.data_)
       data_ = WTF::WrapUnique(new Persistent<T>(other.data_->Get()));
 
     // Invalidated, subsequent mutations will happen on a new copy.
     //
     // (Clearing |m_ownCopy| will not be observable over T, hence
     // the const_cast<> is considered acceptable here.)
     const_cast<DataPersistent&>(other).own_copy_ = false;
@@ skipping 34 matching lines @@
 
  private:
   // Reduce size of DataPersistent<> by delaying creation of Persistent<>.
   std::unique_ptr<Persistent<T>> data_;
   unsigned own_copy_ : 1;
 };
 
 }  // namespace blink
 
 #endif  // DataPersistent_h