header self-sufficiency and idempotency

Change-Id: I4ec74a358e11d85c6eaf1658efd6f1391ee5e387
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/223930
Commit-Queue: Hal Canary <halcanary@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
25 files changed