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.h b/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
index a33a0e6..b601899 100644
--- a/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
+++ b/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
@@ -41,6 +41,8 @@
 
   void SetReservedBitrate(uint32_t reserved_bitrate_bps) override;
 
+  void SetEventLog(RtcEventLog* event_log) override;
+
   int64_t TimeUntilNextProcess() override;
   int32_t Process() override;