Use a blacklist instead of a whitelist for -Wimplicit-fallthrough.

No intended behavior change. Makes it a bit easier to see what's left to do.

Bug: 177475
Change-Id: I314ba2f05db5ebbf8a8cb8cbfaad049f5b34e581
Reviewed-on: https://chromium-review.googlesource.com/902843Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534583}

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_android || is_fuchsia || is_linux || is_win)) {
+  if (is_clang && !is_mac && !is_ios) {
     cflags += [ "-Wimplicit-fallthrough" ]
   }