Revert of Remove SendPacer from ViEEncoder (patchset #13 id:240001 of https://codereview.webrtc.org/1917793002/ )

Reason for revert:
Fails in the waterfall here:

https://build.chromium.org/p/client.webrtc/builders/Win32%20Debug/builds/7832/steps/rtc_media_unittests/logs/stdio

Original issue's description:
> Remove SendPacer from ViEEncoder
>
> This CL moves the logic where the ViEEncoder pause if the pacer is full to the BitrateController. If the queue is full, the controller reports a bitrate of zero to  Call (and BitrateAllocator)
>
> BUG=webrtc:5687
>
> Committed: https://crrev.com/857c5ccdb56e4c94196f7c6227abd5993c95abe2
> Cr-Commit-Position: refs/heads/master@{#12620}

TBR=stefan@webrtc.org,mflodman@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:5687

Review-Url: https://codereview.webrtc.org/1947853002
Cr-Commit-Position: refs/heads/master@{#12621}
diff --git a/webrtc/modules/bitrate_controller/bitrate_controller_impl.h b/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
index 5a61379..a966121 100644
--- a/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
+++ b/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
@@ -28,8 +28,6 @@
 
 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, BitrateObserver* observer);
   virtual ~BitrateControllerImpl() {}
 
@@ -52,11 +50,6 @@
 
   void SetEventLog(RtcEventLog* event_log) override;
 
-  // Returns true if the parameters have changed since the last call.
-  bool GetNetworkParameters(uint32_t* bitrate,
-                            uint8_t* fraction_loss,
-                            int64_t* rtt) override;
-
   int64_t TimeUntilNextProcess() override;
   void Process() override;
 
@@ -71,16 +64,20 @@
                                     int number_of_packets,
                                     int64_t now_ms);
 
-  // Deprecated
   void MaybeTriggerOnNetworkChanged();
 
+  // Returns true if the parameters have changed since the last call.
+  bool GetNetworkParameters(uint32_t* bitrate,
+                            uint8_t* fraction_loss,
+                            int64_t* rtt);
+
   void OnNetworkChanged(uint32_t bitrate,
                         uint8_t fraction_loss,  // 0 - 255.
                         int64_t rtt) EXCLUSIVE_LOCKS_REQUIRED(critsect_);
 
   // Used by process thread.
-  Clock* const clock_;
-  BitrateObserver* const observer_;
+  Clock* clock_;
+  BitrateObserver* observer_;
   int64_t last_bitrate_update_ms_;
 
   rtc::CriticalSection critsect_;