Move BitrateAllocation to api/ and rename it VideoBitrateAllocation

Since the webrtc_common build target does not have visibility set, we
cannot easily use BitrateAllocation in other parts of Chromium.
This is currently blocking parts of chromium:794608, and I know of other
usage outside webrtc already, so moving it to api/ should be warranted.

Also, since there's some naming confusion and this class is video
specific rename it VideoBitrateAllocation. This also fits with the
standard interface for producing these: VideoBitrateAllocator.

Bug: chromium:794608
Change-Id: I4c0fae40f9365e860c605a76a4f67ecc9b9cf9fe
Reviewed-on: https://webrtc-review.googlesource.com/70783
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22986}

video/video_send_stream_impl.h

@@ -126,7 +126,8 @@ class VideoSendStreamImpl : public webrtc::BitrateAllocatorObserver,
       const RTPFragmentationHeader* fragmentation) override;
 
   // Implements VideoBitrateAllocationObserver.
-  void OnBitrateAllocationUpdated(const BitrateAllocation& allocation) override;
+  void OnBitrateAllocationUpdated(
+      const VideoBitrateAllocation& allocation) override;
 
   // Starts monitoring and sends a keyframe.
   void StartupVideoSendStream();