Verifies trials are populated when creating a Call.

This check just makes it more clear what the expectations are.

Pululating trials was made mandatory in an earlier CL, but if you don't
populate this field it will trigger a DCHECK at lower layer where we're
actually trying to parse an experiment. That is confusing and
misleading.

Bug: None
Change-Id: I1f520841a5a3b911048c8ee6d309eb7bb179e037
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/161301
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30005}

call/call.cc

@@ -465,6 +465,7 @@ Call::Call(Clock* clock,
       transport_send_ptr_(transport_send.get()),
       transport_send_(std::move(transport_send)) {
   RTC_DCHECK(config.event_log != nullptr);
+  RTC_DCHECK(config.trials != nullptr);
   worker_sequence_checker_.Detach();
 
   call_stats_->RegisterStatsObserver(&receive_side_cc_);