Mojo: Convert scoped_ptr<...>() to nullptr in mojo/embedder.

mojo/embedder/simple_platform_shared_buffer_win.cc

 // Copyright 2014 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 "mojo/embedder/simple_platform_shared_buffer.h"
 
 #include <windows.h>
 
 #include <limits>
 
@@ skipping 57 matching lines @@
   DCHECK_LE(static_cast<uint64_t>(real_offset),
             static_cast<uint64_t>(std::numeric_limits<DWORD>::max()));
 
   void* real_base = MapViewOfFile(handle_.get().handle,
                                   FILE_MAP_READ | FILE_MAP_WRITE,
                                   0,
                                   static_cast<DWORD>(real_offset),
                                   real_length);
   if (!real_base) {
     PLOG(ERROR) << "MapViewOfFile";
-    return scoped_ptr<PlatformSharedBufferMapping>();
+    return nullptr;
   }
 
   void* base = static_cast<char*>(real_base) + offset_rounding;
   return scoped_ptr<PlatformSharedBufferMapping>(
       new SimplePlatformSharedBufferMapping(
           base, length, real_base, real_length));
 }
 
 // SimplePlatformSharedBufferMapping -------------------------------------------
 
 void SimplePlatformSharedBufferMapping::Unmap() {
   BOOL result = UnmapViewOfFile(real_base_);
   PLOG_IF(ERROR, !result) << "UnmapViewOfFile";
 }
 
 }  // namespace embedder
 }  // namespace mojo