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

mojo/embedder/simple_platform_shared_buffer_posix.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 <stdint.h>
 #include <stdio.h>     // For |fileno()|.
 #include <sys/mman.h>  // For |mmap()|/|munmap()|.
 #include <sys/stat.h>
@@ skipping 119 matching lines @@
   void* real_base = mmap(nullptr,
                          real_length,
                          PROT_READ | PROT_WRITE,
                          MAP_SHARED,
                          handle_.get().fd,
                          static_cast<off_t>(real_offset));
   // |mmap()| should return |MAP_FAILED| (a.k.a. -1) on error. But it shouldn't
   // return null either.
   if (real_base == MAP_FAILED || !real_base) {
     PLOG(ERROR) << "mmap";
-    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() {
   int result = munmap(real_base_, real_length_);
   PLOG_IF(ERROR, result != 0) << "munmap";
 }
 
 }  // namespace embedder
 }  // namespace mojo