Move BitrateAllocator reference from ViEEncoder to VideoSendStream.

This CL will be followed up with a CL adding AudioSendStream to
BitrateAllocator, so this is a small CL to have the video connection to
BitrateAllocator "at the same level" as for audio.

BUG=webrtc:5079
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1785283002 .

Cr-Commit-Position: refs/heads/master@{#11955}
diff --git a/webrtc/call/bitrate_allocator.h b/webrtc/call/bitrate_allocator.h
index 5028e12..25ca735 100644
--- a/webrtc/call/bitrate_allocator.h
+++ b/webrtc/call/bitrate_allocator.h
@@ -6,10 +6,6 @@
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
- *
- *  Usage: this class will register multiple RtcpBitrateObserver's one at each
- *  RTCP module. It will aggregate the results and run one bandwidth estimation
- *  and push the result to the encoders via BitrateObserver(s).
  */
 
 #ifndef WEBRTC_CALL_BITRATE_ALLOCATOR_H_
@@ -25,13 +21,26 @@
 
 namespace webrtc {
 
-class BitrateObserver;
+// Used by all send streams with adaptive bitrate, to get the currently
+// allocated bitrate for the send stream. The current network properties are
+// given at the same time, to let the send stream decide about possible loss
+// protection.
+class BitrateAllocatorObserver {
+ public:
+  virtual void OnBitrateUpdated(uint32_t bitrate_bps,
+                                uint8_t fraction_loss,
+                                int64_t rtt) = 0;
+  virtual ~BitrateAllocatorObserver() {}
+};
 
+// Usage: this class will register multiple RtcpBitrateObserver's one at each
+// RTCP module. It will aggregate the results and run one bandwidth estimation
+// and push the result to the encoders via BitrateAllocatorObserver(s).
 class BitrateAllocator {
  public:
   BitrateAllocator();
 
-  // Allocate target_bitrate across the registered BitrateObservers.
+  // Allocate target_bitrate across the registered BitrateAllocatorObservers.
   // Returns actual bitrate allocated (might be higher than target_bitrate if
   // for instance EnforceMinBitrate() is enabled.
   uint32_t OnNetworkChanged(uint32_t target_bitrate,
@@ -44,11 +53,11 @@
   // |min_bitrate_bps| = 0 equals no min bitrate.
   // |max_bitrate_bps| = 0 equals no max bitrate.
   // Returns bitrate allocated for the bitrate observer.
-  int AddBitrateObserver(BitrateObserver* observer,
-                         uint32_t min_bitrate_bps,
-                         uint32_t max_bitrate_bps);
+  int AddObserver(BitrateAllocatorObserver* observer,
+                  uint32_t min_bitrate_bps,
+                  uint32_t max_bitrate_bps);
 
-  void RemoveBitrateObserver(BitrateObserver* observer);
+  void RemoveObserver(BitrateAllocatorObserver* observer);
 
   void GetMinMaxBitrateSumBps(int* min_bitrate_sum_bps,
                               int* max_bitrate_sum_bps) const;
@@ -68,19 +77,20 @@
     uint32_t max_bitrate;
   };
   struct ObserverConfiguration {
-    ObserverConfiguration(BitrateObserver* observer, uint32_t bitrate)
+    ObserverConfiguration(BitrateAllocatorObserver* observer, uint32_t bitrate)
         : observer(observer), min_bitrate(bitrate) {}
-    BitrateObserver* const observer;
+    BitrateAllocatorObserver* const observer;
     uint32_t min_bitrate;
   };
-  typedef std::pair<BitrateObserver*, BitrateConfiguration>
+  typedef std::pair<BitrateAllocatorObserver*, BitrateConfiguration>
       BitrateObserverConfiguration;
   typedef std::list<BitrateObserverConfiguration> BitrateObserverConfList;
   typedef std::multimap<uint32_t, ObserverConfiguration> ObserverSortingMap;
-  typedef std::map<BitrateObserver*, int> ObserverBitrateMap;
+  typedef std::map<BitrateAllocatorObserver*, int> ObserverBitrateMap;
 
   BitrateObserverConfList::iterator FindObserverConfigurationPair(
-      const BitrateObserver* observer) EXCLUSIVE_LOCKS_REQUIRED(crit_sect_);
+      const BitrateAllocatorObserver* observer)
+      EXCLUSIVE_LOCKS_REQUIRED(crit_sect_);
   ObserverBitrateMap AllocateBitrates() EXCLUSIVE_LOCKS_REQUIRED(crit_sect_);
   ObserverBitrateMap NormalRateAllocation(uint32_t bitrate,
                                           uint32_t sum_min_bitrates)