Mark |Clock*| as |const Clock*| (for some CongestionController and BWE related modules)

webrtc/modules/bitrate_controller/bitrate_controller_impl.h

Index: webrtc/modules/bitrate_controller/bitrate_controller_impl.h
diff --git a/webrtc/modules/bitrate_controller/bitrate_controller_impl.h b/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
index 7ee6b19380a2ccfb6ef011d732cfcbba0175978a..1a53e5d3749f70244349101e5a2c922ca14d6513 100644
--- a/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
+++ b/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
@@ -31,7 +31,7 @@ class BitrateControllerImpl : public BitrateController {
  public:
   // TODO(perkj): BitrateObserver has been deprecated and is not used in WebRTC.
   // |observer| is left for project that is not yet updated.
-  BitrateControllerImpl(Clock* clock,
+  BitrateControllerImpl(const Clock* clock,
                         BitrateObserver* observer,
                         RtcEventLog* event_log);
   virtual ~BitrateControllerImpl() {}
@@ -85,7 +85,7 @@ class BitrateControllerImpl : public BitrateController {
                         int64_t rtt) EXCLUSIVE_LOCKS_REQUIRED(critsect_);
 
   // Used by process thread.
-  Clock* const clock_;
+  const Clock* const clock_;
   BitrateObserver* const observer_;
   int64_t last_bitrate_update_ms_;
   RtcEventLog* const event_log_;