Add methods to help calculate a stack's resources

In order to limit a stack's size in future changes, we need to know
how many resources are already in the stack. We also need to be able
to find the root stack object of a nested stack.

Change-Id: Ib848bcd2e10d02dffc30dce45a2675a9f718fa7d
Related-Bug: #1215100
Merge "Add methods to help calculate a stack's resources"