Enable DSCP by default.

DSCP is controlled by the spec-compliant API
RTCRtpEncodingParameters.networkPriority[1]. It already has a default
value that is the same as when DSCP is disabled.
- If you want non-default DSCP default values, you need to set
  networkPriority and shouldn't need to set a non-standard googDscp flag
  for it to have an effect.
- If you want the default DSCP value, you wouldn't change
  networkPriority and so you don't care if enable_dscp is true... you'll
  get the default regardless.

Drive-by: This CL also adds crbug references to other goog flags.

[1] https://w3c.github.io/webrtc-priority/#dom-rtcrtpencodingparameters-networkpriority

Bug: chromium:1315574
Change-Id: I15a0470fa04f55e2534cee0d240eeb03446c2de6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/258940
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36550}

api/peer_connection_interface.h

@@ -428,8 +428,8 @@ class RTC_EXPORT PeerConnectionInterface : public rtc::RefCountInterface {
     //////////////////////////////////////////////////////////////////////////
 
     // If set to true, don't gather IPv6 ICE candidates.
-    // TODO(deadbeef): Remove this? IPv6 support has long stopped being
-    // experimental
+    // TODO(https://crbug.com/1315576): Remove the ability to set it in Chromium
+    // and delete this flag.
     bool disable_ipv6 = false;
 
     // If set to true, don't gather IPv6 ICE candidates on Wi-Fi.