Fix -Wimplicit-fallthrough build after...

Fix -Wimplicit-fallthrough build after https://chromium-review.googlesource.com/c/chromium/src/+/885085

TBR=metzman

Bug: 177475
Change-Id: I331bc2d1bac790133cbba8ba61ab02e04243a3a6
Reviewed-on: https://chromium-review.googlesource.com/897745Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533661}

testing/libfuzzer/proto/skia_image_filter_proto_converter.cc

@@ -1302,7 +1302,7 @@ void Converter::Visit(const PathRef& path_ref) {
           break;
         case ValidVerb::kConic_Verb:
           num_conics += 1;
-        // fall-through
+          FALLTHROUGH;
         case ValidVerb::kQuad_Verb:
           num_points += 2;
           break;