ahat: Show GC Root Paths.

The Dominator Path in the objects view is replaced by an augmented
Sample Path from GC Root, which includes non-dominator objects
along a sample path and field names.

Also, use blanks instead of "0" in heap tables when the size is 0.
This cleans up the pages a little, and conveniently lets us
distinguish between dominator and non-dominator objects in the Sample
Path from GC Root.

Test: m ahat-test, with new InstanceUtils.gcRootPath test added.

Bug: 27299030
Change-Id: I53d75f9dcb3157c2b5b3afc74958711536cd67b6