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/rtp_rtcp/source/rtcp_sender.cc

@@ -947,7 +947,8 @@ bool RTCPSender::AllVolatileFlagsConsumed() const {
   return true;
 }
 
-void RTCPSender::SetVideoBitrateAllocation(const BitrateAllocation& bitrate) {
+void RTCPSender::SetVideoBitrateAllocation(
+    const VideoBitrateAllocation& bitrate) {
   rtc::CritScope lock(&critical_section_rtcp_sender_);
   video_bitrate_allocation_.emplace(bitrate);
   SetFlag(kRtcpAnyExtendedReports, true);