Add dependency visiting methods to BottomUpMutatorContext
Add dependency visiting methods to BottomUpMutatorContext

It can be useful for a BottomUpMutator to visit its dependencies.  Embed
all of TopDownMutatorContext into BottomUpMutatorContext.

Change-Id: I7c7262fbe3d8fa9cc0d26899eee704cc862835df

danw/blueprint,danw/blueprint,google/blueprint,google/blueprint