Add UMA for tracking which BWE versions are in use.

webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_single_stream.cc

Index: webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_single_stream.cc
diff --git a/webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_single_stream.cc b/webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_single_stream.cc
index d97bb4178423b6b8ed8b54dfae8cc337fb2ee0d9..b1e478f1f3cd1fc93885989753391e5f41ac86f6 100644
--- a/webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_single_stream.cc
+++ b/webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_single_stream.cc
@@ -21,6 +21,7 @@
 #include "webrtc/modules/remote_bitrate_estimator/overuse_estimator.h"
 #include "webrtc/system_wrappers/include/clock.h"
 #include "webrtc/system_wrappers/include/critical_section_wrapper.h"
+#include "webrtc/system_wrappers/include/metrics.h"
 #include "webrtc/typedefs.h"
 
 namespace webrtc {
@@ -54,7 +55,8 @@ RemoteBitrateEstimatorSingleStream::RemoteBitrateEstimatorSingleStream(
       observer_(observer),
       crit_sect_(CriticalSectionWrapper::CreateCriticalSection()),
       last_process_time_(-1),
-      process_interval_ms_(kProcessIntervalMs) {
+      process_interval_ms_(kProcessIntervalMs),
+      uma_recorded_(false) {
   assert(observer_);
   LOG(LS_INFO) << "RemoteBitrateEstimatorSingleStream: Instantiating.";
 }
@@ -71,6 +73,14 @@ void RemoteBitrateEstimatorSingleStream::IncomingPacket(
     int64_t arrival_time_ms,
     size_t payload_size,
     const RTPHeader& header) {
+  if (!uma_recorded_) {
+    BweNames type = BweNames::kReceiverTOffset;
+    if (!header.extension.hasTransmissionTimeOffset)
+      type = BweNames::kReceiverNoExtension;
+    RTC_LOGGED_HISTOGRAM_ENUMERATION(
+        kBweTypeHistogram, type, BweNames::kBweNamesMax);
+    uma_recorded_ = true;
+  }
   uint32_t ssrc = header.ssrc;
   uint32_t rtp_timestamp = header.timestamp +
       header.extension.transmissionTimeOffset;