RemoteBitrateEstimatorAbsSendTime: check clock is a valid ref

Bug: webrtc:8607
Change-Id: Idc3b6c0b3896381f0140584d8c2952ee26db1646
Reviewed-on: https://webrtc-review.googlesource.com/31320
Commit-Queue: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21623}
diff --git a/AUTHORS b/AUTHORS
index 0e934f9..6fe77482 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -80,3 +80,4 @@
 The WebRTC Authors <*@webrtc.org>
 Vonage Holdings Corp. <*@vonage.com>
 Wire Swiss GmbH <*@wire.com>
+Miguel Paris <mparisdiaz@gmail.com>
diff --git a/modules/remote_bitrate_estimator/remote_bitrate_estimator_abs_send_time.cc b/modules/remote_bitrate_estimator/remote_bitrate_estimator_abs_send_time.cc
index f020b21..fdc1523 100644
--- a/modules/remote_bitrate_estimator/remote_bitrate_estimator_abs_send_time.cc
+++ b/modules/remote_bitrate_estimator/remote_bitrate_estimator_abs_send_time.cc
@@ -91,6 +91,7 @@
         first_packet_time_ms_(-1),
         last_update_ms_(-1),
         uma_recorded_(false) {
+    RTC_DCHECK(clock_);
     RTC_DCHECK(observer_);
     RTC_LOG(LS_INFO) << "RemoteBitrateEstimatorAbsSendTime: Instantiating.";
 }