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

This reverts commit 825eb58d59940a4c3c9837595c4b3b07059c93ca.

This Relands the cl reviewed in https://codereview.webrtc.org/1917793002/

patchset #1 is a pure reland.
patchset #2 fix an overflow in BitrateProber that caused WebRtcVideoChannel2BaseTest.TwoStreamsSendAndReceive to fail.

Original cl 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)

R=stefan@webrtc.org
TBR=mflodman@webrtc.org

BUG=webrtc:5687

Review URL: https://codereview.webrtc.org/1947873002 .

Cr-Commit-Position: refs/heads/master@{#12630}
diff --git a/webrtc/modules/bitrate_controller/bitrate_controller_impl.h b/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
index a966121..5a61379 100644
--- a/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
+++ b/webrtc/modules/bitrate_controller/bitrate_controller_impl.h
@@ -28,6 +28,8 @@
 
 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() {}
 
@@ -50,6 +52,11 @@
 
   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;
 
@@ -64,20 +71,16 @@
                                     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* clock_;
-  BitrateObserver* observer_;
+  Clock* const clock_;
+  BitrateObserver* const observer_;
   int64_t last_bitrate_update_ms_;
 
   rtc::CriticalSection critsect_;