Vulkan: Move the CommandGraphNode class to the cpp. This totally hides the details of the CommandGraphNode implementation from the rest of the back-end. This continues the simplification of the graph/resource APIs. Refactoring change only. Bug: angleproject:2539 Change-Id: I7e0f286c387599624cfdff6c8972a8e082fe05d3 Reviewed-on: https://chromium-review.googlesource.com/1052069 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Luc Ferron <lucferron@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org>