Vulkan: Add CommandGraph class. 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/922121 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>