Update style guidance for [[fallthrough]].

We now use compiler-enforced fallthrough annotations.
(http://review.skia.org/296938)

Change-Id: Ia02e0f6147118f7b388315341ed41f60c53b429c
No-Try: true
Docs-Preview: https://skia.org/?cl=309045
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/309045
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed