graph_constructor: Cosmetic changes.

My intent is to be able to re-use most of GraphConstructor to implement the
equivalent of python's import_graph_def() in C++. Breaking that intent
down into a series of smaller changes that morph GraphConstructor into
a form that will work for that, as well as continue to serve the purpose
it does today.

This particular change should not change behavior in any way.
Change: 133456212