Work on getting things built with -Wimplicit-fallthrough. This CL was uploaded by git cl split.

Bug: 177475
R=wuchengli@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I3a2821e16c61556247324c867daaa008cf7f61d6
Reviewed-on: https://chromium-review.googlesource.com/889788Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532276}

media/gpu/h264_decoder.cc

@@ -1342,7 +1342,7 @@ H264Decoder::DecodeResult H264Decoder::Decode() {
         if (state_ != kDecoding)
           break;
 
-      // else fallthrough
+        FALLTHROUGH;
       case H264NALU::kIDRSlice: {
         // TODO(posciak): the IDR may require an SPS that we don't have
         // available. For now we'd fail if that happens, but ideally we'd like