Adds a "tag" system so that entries of dawn.json can be conditionally
emitted in different configurations. With a few more dawn.json changes,
this will enable generating the exact upstream header.
Bug: dawn:1080
Change-Id: I3506dadd485e31786578a3a64c3603c964c5354f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/62580
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>