Include video_bitrate_allocator.h, now that's in api/

Bug: webrtc:10198
Change-Id: Ib2e825d9ddf41d6b3588787ad3aa37a62c81b1a4
Reviewed-on: https://webrtc-review.googlesource.com/c/118922
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26354}

api/video/video_stream_encoder_interface.h

@@ -14,6 +14,7 @@
 #include <vector>
 
 #include "api/rtp_parameters.h"  // For DegradationPreference.
+#include "api/video/video_bitrate_allocator.h"
 #include "api/video/video_sink_interface.h"
 #include "api/video/video_source_interface.h"
 #include "api/video_codecs/video_encoder.h"
@@ -21,9 +22,6 @@
 
 namespace webrtc {
 
-// TODO(nisse): Move full declaration to api/.
-class VideoBitrateAllocationObserver;
-
 // This interface represents a class responsible for creating and driving the
 // encoder(s) for a single video stream. It is also responsible for adaptation
 // decisions related to video quality, requesting reduced frame rate or