Enable implicit-fallthrough warnings on Clang.

Change-Id: I5c1e6d8fd52ecfe628a78569b4665a64e1499fa5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/296938
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
3 files changed