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}

call/call_perf_tests.cc

diff --git a/call/call_perf_tests.cc b/call/call_perf_tests.cc
index 6520e6f..ba48d9a 100644
--- a/call/call_perf_tests.cc
+++ b/call/call_perf_tests.cc
@@ -716,7 +716,7 @@
       return FakeEncoder::InitEncode(config, number_of_cores, max_payload_size);
     }
 
-    int32_t SetRateAllocation(const BitrateAllocation& rate_allocation,
+    int32_t SetRateAllocation(const VideoBitrateAllocation& rate_allocation,
                               uint32_t framerate) override {
       last_set_bitrate_kbps_ = rate_allocation.get_sum_kbps();
       if (encoder_inits_ == 1 &&