GN: Add -Wimplicit-fallthrough warning

webrtc/BUILD.gn

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 # TODO(kjellander): Rebase this to webrtc/build/common.gypi changes after r6330.
 
@@ skipping 96 matching lines @@
         cflags_cc = [
           "-Wnon-virtual-dtor",
 
           # This is enabled for clang; enable for gcc as well.
           "-Woverloaded-virtual",
         ]
       }
     }
 
     if (is_clang) {
-      cflags += [ "-Wthread-safety" ]
+      cflags += [
+        "-Wimplicit-fallthrough",
+        "-Wthread-safety",
+      ]
     }
   }
 
   if (current_cpu == "arm64") {
     defines += [ "WEBRTC_ARCH_ARM64" ]
     defines += [ "WEBRTC_HAS_NEON" ]
   }
 
   if (current_cpu == "arm") {
     defines += [ "WEBRTC_ARCH_ARM" ]
@@ skipping 153 matching lines @@
 if (use_libfuzzer || use_drfuzz) {
   # This target is only here for gn to discover fuzzer build targets under
   # webrtc/test/fuzzers/.
   group("webrtc_fuzzers_dummy") {
     testonly = true
     deps = [
       "test/fuzzers:webrtc_fuzzer_main",
     ]
   }
 }