Add UMA logging of WebRTC usage patterns.

This adds (as a drive-by) logging when unknown values are
used in UMA-from-webrtc, since this indicates an incomplete change.

Bug: chromium:718508
Change-Id: Ic21551e677a58f34d8e2418c17d1bf3256928cdb
Reviewed-on: https://chromium-review.googlesource.com/1082476Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566403}

content/renderer/media/webrtc/rtc_peer_connection_handler.cc

@@ -685,7 +685,13 @@ class PeerConnectionUMAObserver : public webrtc::UMAObserver {
         break;
       default:
         // The default clause is expected to be reached when new enum types are
-        // added.
+        // added. Log, but only once.
+        {
+          static bool log_once = false;
+          LOG_IF(ERROR, !log_once) << "New WebRTC enum counter found: "
+                                   << static_cast<int>(counter_type);
+          log_once = true;
+        }
         break;
     }
   }
@@ -726,9 +732,18 @@ class PeerConnectionUMAObserver : public webrtc::UMAObserver {
         base::UmaHistogramSparse("WebRTC.PeerConnection.SrtcpUnprotectError",
                                  counter);
         break;
+      case webrtc::kEnumCounterUsagePattern:
+        base::UmaHistogramSparse("WebRTC.PeerConnection.UsagePattern", counter);
+        break;
       default:
-        // The default clause is expected to reach when new enum types are
-        // added.
+        // The default clause is expected to be reached when new enum types are
+        // added. Log, but only once.
+        {
+          static bool log_once = false;
+          LOG_IF(ERROR, !log_once) << "New WebRTC sparse enum counter found: "
+                                   << static_cast<int>(counter_type);
+          log_once = true;
+        }
         break;
     }
   }
@@ -751,8 +766,14 @@ class PeerConnectionUMAObserver : public webrtc::UMAObserver {
                                  value);
         break;
       default:
-        // The default clause is expected to reach when new enum types are
-        // added.
+        // The default clause is expected to be reached when new enum types are
+        // added. Log, but only once.
+        {
+          static bool log_once = false;
+          LOG_IF(ERROR, !log_once) << "New WebRTC histogram counter found: "
+                                   << static_cast<int>(type);
+          log_once = true;
+        }
         break;
     }
   }

tools/metrics/histograms/enums.xml

@@ -48318,6 +48318,16 @@ Full version information for the fingerprint enum values:
   <int value="4" label="Other"/>
 </enum>
 
+<enum name="WebRtcPeerConnectionUsagePattern">
+  <summary>
+    These values are integers representing a set of flags indicating how the
+    PeerConnection is used. For flag values, see header file
+    webrtc/pc/peerconnection.h, enum class UsageEvent. Only interesting values
+    are called out with labels.
+  </summary>
+  <int value="0" label="Unused and unclosed"/>
+</enum>
+
 <enum name="WebRtcVideoCodecs">
   <int value="0" label="Unknown"/>
   <int value="1" label="VP8"/>

tools/metrics/histograms/histograms.xml

@@ -109529,6 +109529,16 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.PeerConnection.UsagePattern"
+    enum="WebRtcPeerConnectionUsagePattern">
+  <owner>hta@chromium.org</owner>
+  <summary>
+    Capsule history of a WebRTC PeerConnection, encoded as a sequence of bits
+    encapsulated in an integer. Only a few values will be deemed interesting,
+    but the interesting values may change over time.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.ReceivedAudioTrackDuration" units="ms">
   <owner>perkj@chromium.org</owner>
   <summary>