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

mojo/embedder/simple_platform_shared_buffer_posix.cc

Index: mojo/embedder/simple_platform_shared_buffer_posix.cc
diff --git a/mojo/embedder/simple_platform_shared_buffer_posix.cc b/mojo/embedder/simple_platform_shared_buffer_posix.cc
index 9a111735eef9b97512c75373512b88b64fb8af1a..f6af3f56d5c6b4ec3e65b0ccc0729c6d2cb12517 100644
--- a/mojo/embedder/simple_platform_shared_buffer_posix.cc
+++ b/mojo/embedder/simple_platform_shared_buffer_posix.cc
@@ -137,7 +137,7 @@
   // 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;