Teach `benchmark.mojo` to save the collected trace file to disk.

apps/benchmark/run_args.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "apps/benchmark/run_args.h"
 
 #include <algorithm>
 
 #include "base/command_line.h"
 #include "base/logging.h"
@@ skipping 50 matching lines @@
     return false;
   }
   int duration_int;
   if (!base::StringToInt(duration_str, &duration_int) || duration_int <= 0) {
     LOG(ERROR) << "Could not parse the --duration value as a positive integer: "
                << duration_str;
     return false;
   }
   result->duration = base::TimeDelta::FromSeconds(duration_int);
 
+  result->write_output_file = false;
+  if (command_line.HasSwitch("trace-output")) {
+    result->write_output_file = true;
+    result->output_file_path =
+        base::FilePath(command_line.GetSwitchValueASCII("trace-output"));
+  }
+
   // All regular arguments (not switches, ie. not preceded by "--") describe
   // measurements.
   for (const std::string& measurement_spec : command_line.GetArgs()) {
     Measurement measurement;
     if (!GetMeasurement(measurement_spec, &measurement)) {
       return false;
     }
     result->measurements.push_back(measurement);
   }
 
   return true;
 }
 
 }  // namespace benchmark