Enable -Wimplicit-fallthrough on chromecast.

This Just Works after https://chromium-review.googlesource.com/897393

Bug: 177475
Change-Id: I5769827692dfc4f864db403b3e525c9afb00436b
Reviewed-on: https://chromium-review.googlesource.com/899114Reviewed-by: Stephen Lanham <slan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534159}

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_chromecast && !use_ozone))) {
+  if (is_clang && (is_android || is_fuchsia || (is_linux && !use_ozone))) {
     cflags += [ "-Wimplicit-fallthrough" ]
   }