Also updates relevant comments to no longer refer to a graph.

Bug: angleproject:4029
Change-Id: Ic29716e9ae4926870f902947d49d8fee7af98662
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2057804
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>