This also renames CommandBufferNode to CommandGraphNode. It also
renames some of the intenal members to more closely represent the
tree relationships (parents/children). This should clean up the
command graph classes and make them a bit easier to understand.

Bug: angleproject:2361
Change-Id: I024bffcc7f4157c78072ef902a3c40a07a08b18a
Reviewed-on: https://chromium-review.googlesource.com/922121Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>