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}

modules/video_coding/generic_encoder.cc

@@ -38,8 +38,9 @@ VCMGenericEncoder::VCMGenericEncoder(
     : encoder_(encoder),
       vcm_encoded_frame_callback_(encoded_frame_callback),
       internal_source_(internal_source),
-      encoder_params_({BitrateAllocation(), 0, 0, 0}),
-      streams_or_svc_num_(0) {}
+      encoder_params_({VideoBitrateAllocation(), 0, 0, 0}),
+      streams_or_svc_num_(0),
+      codec_type_(VideoCodecType::kVideoCodecUnknown) {}
 
 VCMGenericEncoder::~VCMGenericEncoder() {}