Ship GenericDescriptor00 by default.

The change ships GenericDescriptor00 and authentication by default,
but doesn't expose it by default, and makes WebRTC respond to
offers carrying it.

The change adds a unit test for the new semantics.

Tests well in munge-sdp. Frame marking replaced by
http://www.webrtc.org/experiments/rtp-hdrext/generic-frame-descriptor-00
in the offer results in an answer containing the
extension as first entry.

Bug: webrtc:11367
Change-Id: I0ef91b7d4096d949c3d547ece7d6c4d39aa241da
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168661
Reviewed-by: Magnus Flodman <mflodman@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Markus Handell <handellm@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30542}
diff --git a/modules/rtp_rtcp/source/rtp_sender_video.cc b/modules/rtp_rtcp/source/rtp_sender_video.cc
index 2696514..78ece7f 100644
--- a/modules/rtp_rtcp/source/rtp_sender_video.cc
+++ b/modules/rtp_rtcp/source/rtp_sender_video.cc
@@ -269,7 +269,7 @@
       require_frame_encryption_(config.require_frame_encryption),
       generic_descriptor_auth_experiment_(
           config.field_trials->Lookup("WebRTC-GenericDescriptorAuth")
-              .find("Enabled") == 0),
+              .find("Disabled") != 0),
       exclude_transport_sequence_number_from_fec_experiment_(
           config.field_trials
               ->Lookup(kExcludeTransportSequenceNumberFromFecFieldTrial)
@@ -656,7 +656,7 @@
 
     size_t bytes_written = 0;
 
-    // Only enable header authentication if the field trial is enabled.
+    // Enable header authentication if the field trial isn't disabled.
     rtc::ArrayView<const uint8_t> additional_data;
     if (generic_descriptor_auth_experiment_) {
       additional_data = generic_descriptor_raw;