Also adds a more consistent way of checking if the debug utils
extension is enabled.

Enables adding support for the debug utils markers with the command
graph disabled.

Bug: angleproject:4029
Change-Id: I5f8762921b06f54e400c25764012ab70e10bfb8d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2055554Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>