Add VP8 and H264 depacketizer fuzzers.

Also removes listing of targets in webrtc_fuzzers which is very prone to
not being up to date. They're not required for ClusterFuzz integration
or building locally. This also means that adding fuzzers won't require
approval outside the fuzzers directory.

BUG=webrtc:4771
R=kjellander@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1518973003 .

Cr-Commit-Position: refs/heads/master@{#11067}
diff --git a/webrtc/BUILD.gn b/webrtc/BUILD.gn
index f11509c..b26d29f 100644
--- a/webrtc/BUILD.gn
+++ b/webrtc/BUILD.gn
@@ -281,15 +281,12 @@
 }
 
 if (use_libfuzzer || use_drfuzz) {
-  group("webrtc_fuzzers") {
+  # 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:audio_decoder_ilbc_fuzzer",
-      "test/fuzzers:audio_decoder_isac_fuzzer",
-      "test/fuzzers:audio_decoder_isacfix_fuzzer",
-      "test/fuzzers:audio_decoder_opus_fuzzer",
-      "test/fuzzers:vp8_qp_parser_fuzzer",
-      "test/fuzzers:vp9_depacketizer_fuzzer",
+      "test/fuzzers:webrtc_fuzzer_main",
     ]
   }
 }