Fix FALLTHROUGH for C++17 case
Fix FALLTHROUGH for C++17 case

After
https://chromium-review.googlesource.com/c/chromium/mini_chromium/+/899847

Change-Id: I117cf06c3a91acdbafbd6cdf633d293f4d550a19
Reviewed-on: https://chromium-review.googlesource.com/900524
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>

chromium/mini_chromium,chromium/mini_chromium,chromium/mini_chromium