Mojo: NULL -> nullptr in mojo/public/cpp outside of bindings.

mojo/public/cpp/system/buffer.h

Index: mojo/public/cpp/system/buffer.h
diff --git a/mojo/public/cpp/system/buffer.h b/mojo/public/cpp/system/buffer.h
index 80b03c660964cfd7b824eaaca49a3ad724556691..ad9966a5feea1f4fbdd77b94194698797fa5aaeb 100644
--- a/mojo/public/cpp/system/buffer.h
+++ b/mojo/public/cpp/system/buffer.h
@@ -93,7 +93,7 @@ class SharedBuffer {
 
 inline SharedBuffer::SharedBuffer(uint64_t num_bytes) {
   MojoResult result MOJO_ALLOW_UNUSED =
-      CreateSharedBuffer(NULL, num_bytes, &handle);
+      CreateSharedBuffer(nullptr, num_bytes, &handle);
   assert(result == MOJO_RESULT_OK);
 }