A couple bugs had led code in one Context to be able to lead to
estimated memory usage in another Context, even in cases that should be
easy to detect.

- Ensure that the pointer to the next context is nulled out while
  recursing over the portion of the heap. It seems like there was
  previously some code to do this partway, but the nulling part
  was left out.
- Skip including maps in the understanding of the Context estimated
  size, as the maps are shared between Contexts and may be reachable
  from other Contexts

Review-Url: https://codereview.chromium.org/2780773002
Cr-Commit-Position: refs/heads/master@{#44208}