Refactor bitrate configuration in CallTest

All implementations of ModifyReceiverCallConfig and
ModifySenderCallConfig configure the bitrate_config member only. So
replace these methods by ModifyReceiverBitrateConfig and
ModifySenderBitrateConfig.

This is a preparation for injecting RtpTransportControllerSend via
CallConfig. Then bitrates should be passed when constructing
RtpTransportControllerSend, and they can be deleted from CallConfig.

Bug: webrtc:7135
Change-Id: I6714158bd463dd485018713d0e26815919e5afcc
Reviewed-on: https://webrtc-review.googlesource.com/c/110780
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25624}
diff --git a/call/call_perf_tests.cc b/call/call_perf_tests.cc
index 818b91a..2f935b4 100644
--- a/call/call_perf_tests.cc
+++ b/call/call_perf_tests.cc
@@ -770,8 +770,9 @@
       return FakeEncoder::SetRateAllocation(rate_allocation, framerate);
     }
 
-    void ModifySenderCallConfig(Call::Config* config) override {
-      config->bitrate_config.start_bitrate_bps = kInitialBitrateKbps * 1000;
+    void ModifySenderBitrateConfig(
+        BitrateConstraints* bitrate_config) override {
+      bitrate_config->start_bitrate_bps = kInitialBitrateKbps * 1000;
     }
 
     void ModifyVideoConfigs(