Enable -Wimplicit-fallthrough on Chrome OS.

R=scottmg@chromium.org

Bug: 177475
Change-Id: I830c63748878cc8af7d1b52418a45fe7f05d1734
Reviewed-on: https://chromium-review.googlesource.com/899446Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534520}

build/config/compiler/BUILD.gn

@@ -1429,8 +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_android || is_fuchsia || (is_linux && !is_chromeos) || is_win)) {
+  if (is_clang && (is_android || is_fuchsia || is_linux || is_win)) {
     cflags += [ "-Wimplicit-fallthrough" ]
   }