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

mojo/embedder/simple_platform_shared_buffer_win.cc

Index: mojo/embedder/simple_platform_shared_buffer_win.cc
diff --git a/mojo/embedder/simple_platform_shared_buffer_win.cc b/mojo/embedder/simple_platform_shared_buffer_win.cc
index 1167ac5fa0616b5d5296261de38af65fe79b04ae..97367807efb3796caeb958040d0475ef27d58f15 100644
--- a/mojo/embedder/simple_platform_shared_buffer_win.cc
+++ b/mojo/embedder/simple_platform_shared_buffer_win.cc
@@ -75,7 +75,7 @@
                                   real_length);
   if (!real_base) {
     PLOG(ERROR) << "MapViewOfFile";
-    return scoped_ptr<PlatformSharedBufferMapping>();
+    return nullptr;
   }
 
   void* base = static_cast<char*>(real_base) + offset_rounding;