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>