Change SubRunType to SubRunStreamTag

Make it clear that the enum used by different subruns
is used for tagging the type in byte streams.

Change-Id: Id0f20b38872b420f51c7cf293b3580ba424f63b4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/714736
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
2 files changed