Added protobuf message for loss-based BWE events, and wired it up to the send side bandwidth estimator.

BUG=

Review URL: https://codereview.webrtc.org/1411673003

Cr-Commit-Position: refs/heads/master@{#10531}
diff --git a/webrtc/modules/bitrate_controller/bitrate_controller_impl.cc b/webrtc/modules/bitrate_controller/bitrate_controller_impl.cc
index d779562..f8fd2bb 100644
--- a/webrtc/modules/bitrate_controller/bitrate_controller_impl.cc
+++ b/webrtc/modules/bitrate_controller/bitrate_controller_impl.cc
@@ -129,6 +129,11 @@
   MaybeTriggerOnNetworkChanged();
 }
 
+void BitrateControllerImpl::SetEventLog(RtcEventLog* event_log) {
+  rtc::CritScope cs(&critsect_);
+  bandwidth_estimation_.SetEventLog(event_log);
+}
+
 void BitrateControllerImpl::OnReceivedEstimatedBitrate(uint32_t bitrate) {
   {
     rtc::CritScope cs(&critsect_);