This makes it easier to plumb debug labels to a future
DebugAnnotatorVk class.

Bug: b/162068318
Bug: b/169243237
Change-Id: I01e3779569c27c91252dc2874f6deaec526afd6f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2451516
Commit-Queue: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>