This makes the members private and adds more functionality. This moves
more responsibility out of vk::CommandGraphNode and also makes the
RenderPass init in the CommandGraphNode class better encapsulated.

Bug: angleproject:2539
Change-Id: Ia16f3f39cf011548c6473805b8b28e284808e856
Reviewed-on: https://chromium-review.googlesource.com/1040279Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>