Make header files idempotent; script to check

Change-Id: I960ded854e6bc7cdee029a7393cac2a686c41754
Reviewed-on: https://skia-review.googlesource.com/8308
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
20 files changed