If building a fuzzer, define WEBRTC_UNSAFE_FUZZER_MODE.

This is useful because various pieces of code can then make themselves
more fuzzer-friendly. (For example, checksum verification can always
succeed.) See BORINGSSL_UNSAFE_FUZZER_MODE for an analogous flag.

BUG=chromium:561667

Review-Url: https://codereview.webrtc.org/2000173002
Cr-Commit-Position: refs/heads/master@{#12904}

webrtc/BUILD.gn

@@ -169,6 +169,11 @@ config("common_config") {
       "-fno-builtin-sinf",
     ]
   }
+
+  if (use_libfuzzer || use_drfuzz) {
+    # Used in Chromium's overrides to disable logging
+    defines += [ "WEBRTC_UNSAFE_FUZZER_MODE" ]
+  }
 }
 
 source_set("webrtc") {