Add rtc_enable_external_auth variable for setting ENABLE_EXTERNAL_AUTH

This will make it easier for some downstream projects to control whether
or not to set ENABLE_EXTERNAL_AUTH, via the GN variable.

BUG=None
NOTRY=True

Review-Url: https://codereview.webrtc.org/2608673002
Cr-Commit-Position: refs/heads/master@{#15894}

webrtc/BUILD.gn

@@ -122,13 +122,16 @@ config("common_config") {
     defines += [ "HAVE_SCTP" ]
   }
 
+  if (rtc_enable_external_auth) {
+    defines += [ "ENABLE_EXTERNAL_AUTH" ]
+  }
+
   if (build_with_chromium) {
     defines += [
       # NOTICE: Since common_inherited_config is used in public_configs for our
       # targets, there's no point including the defines in that config here.
       # TODO(kjellander): Cleanup unused ones and move defines closer to the
       # source when webrtc:4256 is completed.
-      "ENABLE_EXTERNAL_AUTH",
       "HAVE_OPENSSL_SSL_H",
       "HAVE_SRTP",
       "HAVE_WEBRTC_VIDEO",

webrtc/build/webrtc.gni

@@ -40,6 +40,10 @@ declare_args() {
   # Disable the code for the intelligibility enhancer by default.
   rtc_enable_intelligibility_enhancer = false
 
+  # Enable when an external authentication mechanism is used for performing
+  # packet authentication for RTP packets instead of libsrtp.
+  rtc_enable_external_auth = build_with_chromium
+
   # Selects whether debug dumps for the audio processing module
   # should be generated.
   apm_debug_dump = false