webrtc: add WebRTC.PeerConnection.SdpFormatReceivedAnswer metrics

similar to WebRTC.PeerConnection.SdpFormatReceived but counting answers.
WebRTC implementation CL:
  https://webrtc-review.googlesource.com/c/src/+/186701

BUG=chromium:857004

Change-Id: Ie09d8bb4a92a4509216351d111b0b3d7c8100e0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2460888
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822119}

tools/metrics/histograms/histograms_xml/web_rtc/histograms.xml

@@ -1310,6 +1310,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 <histogram name="WebRTC.PeerConnection.SdpFormatReceived"
     enum="PeerConnectionSdpFormatReceived" expires_after="2021-04-04">
   <owner>steveanton@chromium.org</owner>
+  <owner>hta@chromium.org</owner>
   <summary>
     What SDP format is received in the remote offer. The value &quot;no
     tracks&quot; means that no audio or video tracks were received. The value
@@ -1318,6 +1319,18 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
     or more than one video track was received, and how this was signaled is
     indicated (&quot;Plan B&quot; meaning with a=ssrc lines within the same m=
     section and &quot;Unified Plan&quot; meaning with a separate m= section).
+    This is recorded when calling setRemoteDescription with an SDP Offer.
+  </summary>
+</histogram>
+
+<histogram name="WebRTC.PeerConnection.SdpFormatReceivedAnswer"
+    enum="PeerConnectionSdpFormatReceived" expires_after="2021-04-04">
+  <owner>steveanton@chromium.org</owner>
+  <owner>hta@chromium.org</owner>
+  <summary>
+    What SDP format is received in the remote answer. See
+    WebRTC.PeerConnection.SdpFormatReceived for the description of the values.
+    This is recorded when calling setRemoteDescription with an SDP Answer.
   </summary>
 </histogram>