Add graph density

Change-Id: I97036e8cf2f528633287d4abe46fe5779a8d5f76
Don't allow mutating operations on the underlying graph

Instead of allowing a direct graph return of the underlying
graph_flow graph we should return a frozen version instead so that
users of the returned value can not mutate the graph without going
through the graph_flow pattern (which could have undesired and
harmful side-effects if this occurs).

Change-Id: I38b35119d6e7bd7387b8ab467eba53aee5500629
Merge "Add graph density"
Merge "Don't allow mutating operations on the underlying graph"