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/test/encoder_proxy_factory.h b/test/encoder_proxy_factory.h
index 9c7ff0c..2982010 100644
--- a/test/encoder_proxy_factory.h
+++ b/test/encoder_proxy_factory.h
@@ -82,7 +82,7 @@
     int32_t SetChannelParameters(uint32_t packet_loss, int64_t rtt) override {
       return encoder_->SetChannelParameters(packet_loss, rtt);
     }
-    int32_t SetRateAllocation(const BitrateAllocation& rate_allocation,
+    int32_t SetRateAllocation(const VideoBitrateAllocation& rate_allocation,
                               uint32_t framerate) override {
       return encoder_->SetRateAllocation(rate_allocation, framerate);
     }