Enable -Wimplicit-fallthrough on all Fuchsia builds.

Unlike the existing Linux warnings, this is enabled for Fuchsia
including Chromecast and Ozone builds.

Bug: 177475, 808138
Change-Id: Idaf85eabbf3029f37c9c37c7b1bd8f03e411a9d1
Reviewed-on: https://chromium-review.googlesource.com/898007
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533940}

build/config/compiler/BUILD.gn

@@ -1429,7 +1429,7 @@ config("chromium_code") {
   # Warn on missing break statements at the end of switch cases.
   # For intentional fallthrough, use FALLTHROUGH; from base/compiler_specific.h
   # TODO(thakis): Enable for all platforms, https://crbug.com/177475
-  if (is_clang && is_linux && !is_chromecast && !use_ozone) {
+  if (is_clang && (is_fuchsia || (is_linux && !is_chromecast && !use_ozone))) {
     cflags += [ "-Wimplicit-fallthrough" ]
   }