What can't loopback test be more like full stack test?

webrtc/video/screenshare_loopback.cc

Index: webrtc/video/screenshare_loopback.cc
diff --git a/webrtc/video/screenshare_loopback.cc b/webrtc/video/screenshare_loopback.cc
index 46f662a45d4a87f0417fe85d845b25ffe24634e8..c0638c9b1aae0099189a6a5f72bed59919786b04 100644
--- a/webrtc/video/screenshare_loopback.cc
+++ b/webrtc/video/screenshare_loopback.cc
@@ -127,7 +127,15 @@ int StdPropagationDelayMs() {
   return static_cast<int>(FLAG_std_propagation_delay_ms);
 }
 
-DEFINE_int(selected_stream, 0, "ID of the stream to show or analyze.");
+DEFINE_int(num_streams, 0, "Number of streams to show or analyze.");
+int NumStreams() {
+  return static_cast<int>(FLAG_num_streams);
+}
+
+DEFINE_int(selected_stream,
+           0,
+           "ID of the stream to show or analyze. "
+           "Set to the number of streams to show them all.");
 int SelectedStream() {
   return static_cast<int>(FLAG_selected_stream);
 }
@@ -268,6 +276,11 @@ void Loopback() {
   params.pipe = pipe_config;
   params.logs = flags::FLAG_logs;
 
+  if (flags::NumStreams() > 1 && flags::Stream0().empty() &&
+      flags::Stream1().empty()) {
+    params.ss.infer_streams = true;
+  }
+
   std::vector<std::string> stream_descriptors;
   stream_descriptors.push_back(flags::Stream0());
   stream_descriptors.push_back(flags::Stream1());
@@ -275,7 +288,7 @@ void Loopback() {
   SL_descriptors.push_back(flags::SL0());
   SL_descriptors.push_back(flags::SL1());
   VideoQualityTest::FillScalabilitySettings(
-      &params, stream_descriptors, flags::SelectedStream(),
+      &params, stream_descriptors, flags::NumStreams(), flags::SelectedStream(),
       flags::NumSpatialLayers(), flags::SelectedSL(), SL_descriptors);
 
   VideoQualityTest test;
@@ -295,8 +308,10 @@ int main(int argc, char* argv[]) {
     return 0;
   }
 
-  webrtc::test::InitFieldTrialsFromString(
-      webrtc::flags::FLAG_force_fieldtrials);
+  // InitFieldTrialsFromString needs a reference to an std::string instance,
+  // with a scope that outlives the test.
+  std::string field_trials = webrtc::flags::FLAG_force_fieldtrials;
+  webrtc::test::InitFieldTrialsFromString(field_trials);
   webrtc::test::RunTest(webrtc::Loopback);
   return 0;
 }