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}
diff --git a/modules/video_coding/generic_encoder.h b/modules/video_coding/generic_encoder.h
index 4efd961..03de626 100644
--- a/modules/video_coding/generic_encoder.h
+++ b/modules/video_coding/generic_encoder.h
@@ -28,7 +28,7 @@
 }  // namespace media_optimization
 
 struct EncoderParameters {
-  BitrateAllocation target_bitrate;
+  VideoBitrateAllocation target_bitrate;
   uint8_t loss_rate;
   int64_t rtt;
   uint32_t input_frame_rate;