[WebRTC] Add deprecation warning when Plan B is used.

Intent to Deprecate on blink-dev:
https://groups.google.com/u/3/a/chromium.org/g/blink-dev/c/IY2amIigFFs

Plan B SDP is still heavily used two years after Unified Plan
shipped-by-default. This can cause compatibility issues with Firefox and
Safari unless the application does sophisticated SDP translation.

This deprecation warning is meant to discourage Plan B and encourage
migration to the standardized SDP format Unified Plan, allowing us to
remove Plan B when usage is sufficiently low.

In the blink-dev discussion we talk about adding adding a Reverse Origin
Trial at a future date in case large services still depend on Plan B
when M93 comes around.

Bug: chromium:857004
Change-Id: Ic95964278a5b7c693cfc7868e957517a64a4ac01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2595744
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843771}

third_party/blink/public/mojom/web_feature/web_feature.mojom

@@ -3091,6 +3091,7 @@ enum WebFeature {
   kAddressSpaceUnknownNonSecureContextNavigatedToLocal = 3770,
   kAddressSpaceUnknownSecureContextNavigatedToPrivate = 3771,
   kAddressSpaceUnknownNonSecureContextNavigatedToPrivate = 3772,
+  kRTCPeerConnectionSdpSemanticsPlanB = 3773,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

third_party/blink/renderer/core/frame/deprecation.cc

@@ -70,6 +70,8 @@ enum Milestone {
   kM89 = 89,
   kM90 = 90,
   kM91 = 91,
+  kM92 = 92,
+  kM93 = 93,
 };
 
 // Returns estimated milestone dates as milliseconds since January 1, 1970.
@@ -130,6 +132,10 @@ base::Time::Exploded MilestoneDate(Milestone milestone) {
       return {2021, 4, 0, 13, 4};
     case kM91:
       return {2021, 5, 0, 25, 4};
+    case kM92:
+      return {2021, 7, 0, 20, 4};
+    case kM93:
+      return {2021, 8, 0, 31, 4};
   }
 
   NOTREACHED();
@@ -555,6 +561,16 @@ DeprecationInfo GetDeprecationInfo(WebFeature feature) {
                   " See https://chromestatus.com/feature/6730096436051968 for "
                   "more details."};
 
+    case WebFeature::kRTCPeerConnectionSdpSemanticsPlanB:
+      return {"RTCPeerConnectionSdpSemanticsPlanB", kM93,
+              "Plan B SDP semantics, which is used when constructing an "
+              "RTCPeerConnection with {sdpSemantics:\"plan-b\"}, is a legacy "
+              "version of the Session Description Protocol that has severe "
+              "compatibility issues on modern browsers. The standardized SDP "
+              "format, \"unified-plan\", has been used by default since M72 "
+              "(January, 2019). Dropping support for Plan B is targeted for "
+              "M93 (Canary: July 15, 2021; Stable: August 24, 2021)."};
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
       return {"NotDeprecated", kUnknown, ""};

third_party/blink/renderer/modules/peerconnection/rtc_peer_connection.cc

@@ -324,6 +324,8 @@ webrtc::PeerConnectionInterface::RTCConfiguration ParseConfiguration(
   if (configuration->hasSdpSemantics()) {
     if (configuration->sdpSemantics() == "plan-b") {
       web_configuration.sdp_semantics = webrtc::SdpSemantics::kPlanB;
+      Deprecation::CountDeprecation(
+          context, WebFeature::kRTCPeerConnectionSdpSemanticsPlanB);
     } else {
       DCHECK_EQ(configuration->sdpSemantics(), "unified-plan");
       web_configuration.sdp_semantics = webrtc::SdpSemantics::kUnifiedPlan;

tools/metrics/histograms/enums.xml

@@ -30967,6 +30967,7 @@ Called by update_use_counter_feature_enum.py.-->
   <int value="3771" label="AddressSpaceUnknownSecureContextNavigatedToPrivate"/>
   <int value="3772"
       label="AddressSpaceUnknownNonSecureContextNavigatedToPrivate"/>
+  <int value="3773" label="RTCPeerConnectionSdpSemanticsPlanB"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">