Add use counters for EnableDtlsSrtp constraint

These will be turned into deprecation warnings when we
start deprecating SDES crypto and the constraint.

Bug: 804275
Change-Id: I0c5763406b9f4bd32890708e34a132ad6553a929
Reviewed-on: https://chromium-review.googlesource.com/941955Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540265}

third_party/WebKit/Source/modules/mediastream/MediaConstraintsImpl.cpp

@@ -381,6 +381,14 @@ static void ParseOldStyleNames(
     } else if (constraint.name_.Equals(kUseRtpMux)) {
       result.goog_use_rtp_mux.SetExact(ToBoolean(constraint.value_));
     } else if (constraint.name_.Equals(kEnableDtlsSrtp)) {
+      bool value = ToBoolean(constraint.value_);
+      if (value) {
+        UseCounter::Count(context,
+                          WebFeature::kRTCConstraintEnableDtlsSrtpTrue);
+      } else {
+        UseCounter::Count(context,
+                          WebFeature::kRTCConstraintEnableDtlsSrtpFalse);
+      }
       result.enable_dtls_srtp.SetExact(ToBoolean(constraint.value_));
     } else if (constraint.name_.Equals(kEnableRtpDataChannels)) {
       result.enable_rtp_data_channels.SetExact(ToBoolean(constraint.value_));

third_party/WebKit/public/platform/web_feature.mojom

@@ -1872,6 +1872,8 @@ enum WebFeature {
   kV8RTCDTMFSender_Duration_AttributeGetter = 2379,
   kV8RTCDTMFSender_InterToneGap_AttributeGetter = 2380,
   kV8RTCRtpSender_Dtmf_AttributeGetter = 2381,
+  kRTCConstraintEnableDtlsSrtpTrue = 2382,
+  kRTCConstraintEnableDtlsSrtpFalse = 2383,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

tools/metrics/histograms/enums.xml

@@ -17904,6 +17904,8 @@ Called by update_net_error_codes.py.-->
   <int value="2379" label="V8RTCDTMFSender_Duration_AttributeGetter"/>
   <int value="2380" label="V8RTCDTMFSender_InterToneGap_AttributeGetter"/>
   <int value="2381" label="V8RTCRtpSender_Dtmf_AttributeGetter"/>
+  <int value="2382" label="RTCConstraintEnableDtlsSrtpTrue"/>
+  <int value="2383" label="RTCConstraintEnableDtlsSrtpFalse"/>
 </enum>
 
 <enum name="FeedbackSource">