Reland "Moved BitrateConfig out of Call::Config."

This is a reland of 5897fe27abcbe70f706cc23adc26147e0581f97e.

Adding back CallConfig::kDefaultStartBitrateBps as deprecated.
Also making BitrateContraints::kDefaultStartBitrateBps private to stop
it from being used in other places.

Original change's description:
> Moved BitrateConfig out of Call::Config.
>
> This prepares for a CL extracting the bitrate configuration logic from
> the Call class.
>
> Also renaming BitrateConfig to BitrateConstraints.
>
> Bug: webrtc:8415
> Change-Id: I7e472683034c57bdc8093cdf5e78e477d1732480
> Reviewed-on: https://webrtc-review.googlesource.com/54400
> Commit-Queue: Sebastian Jansson <srte@webrtc.org>
> Reviewed-by: Stefan Holmer <stefan@webrtc.org>
> Reviewed-by: Niels Moller <nisse@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#22104}

Bug: webrtc:8415
Change-Id: Iacfe2d6daedff710832ab89210c7c66d4403c93b
Reviewed-on: https://webrtc-review.googlesource.com/55980
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22123}
diff --git a/call/call_perf_tests.cc b/call/call_perf_tests.cc
index ddeaf12..e8c3601 100644
--- a/call/call_perf_tests.cc
+++ b/call/call_perf_tests.cc
@@ -888,7 +888,7 @@
 
     void OnCallsCreated(Call* sender_call, Call* receiver_call) override {
       sender_call_ = sender_call;
-      Call::Config::BitrateConfig bitrate_config;
+      BitrateConstraints bitrate_config;
       bitrate_config.min_bitrate_bps = min_bwe_;
       bitrate_config.start_bitrate_bps = start_bwe_;
       bitrate_config.max_bitrate_bps = max_bwe_;