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

apps/benchmark/run_args.h

Index: apps/benchmark/run_args.h
diff --git a/apps/benchmark/run_args.h b/apps/benchmark/run_args.h
index a9ada528d97c61fd528a6be19675de56c33d0677..73e83d6190bbf99074260191da2155f92cca2cb1 100644
--- a/apps/benchmark/run_args.h
+++ b/apps/benchmark/run_args.h
@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "apps/benchmark/measurements.h"
+#include "base/files/file_path.h"
 #include "base/time/time.h"
 
 namespace benchmark {
@@ -18,6 +19,8 @@ struct RunArgs {
   std::string app;
   base::TimeDelta duration;
   std::vector<Measurement> measurements;
+  bool write_output_file;
+  base::FilePath output_file_path;
 
   RunArgs();
   ~RunArgs();