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

apps/benchmark/benchmark_app.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 <memory>
 #include <set>
 #include <string>
 #include <vector>
 
 #include "apps/benchmark/event.h"
 #include "apps/benchmark/measurements.h"
 #include "apps/benchmark/run_args.h"
 #include "apps/benchmark/trace_collector_client.h"
 #include "base/bind.h"
+#include "base/files/file_util.h"
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
 #include "base/time/time.h"
 #include "base/trace_event/trace_event.h"
 #include "mojo/application/application_runner_chromium.h"
 #include "mojo/public/c/system/main.h"
 #include "mojo/public/cpp/application/application_connection.h"
 #include "mojo/public/cpp/application/application_delegate.h"
@@ skipping 51 matching lines @@
   }
 
   void StopTracing() {
     // Request the trace collector to send back the data. When the data is ready
     // we will be called at OnTraceCollected().
     trace_collector_client_->Stop();
   }
 
   // TraceCollectorClient::Receiver:
   void OnTraceCollected(std::string trace_data) override {
+    if (args_.write_output_file) {
+      // Write the trace file regardless of whether it can be parsed (or whether
+      // the measurements succeed), as it can be useful to debug failures.
+      base::File trace_file(
+          args_.output_file_path,
+          base::File::FLAG_CREATE_ALWAYS | base::File::FLAG_WRITE);
+      trace_file.WriteAtCurrentPos(trace_data.data(), trace_data.size());
+      printf("wrote trace file at: %s\n",
+             args_.output_file_path.value().c_str());
+    }
+
     // Parse trace events.
     std::vector<Event> events;
     if (!GetEvents(trace_data, &events)) {
       LOG(ERROR) << "Failed to parse the trace data";
       mojo::ApplicationImpl::Terminate();
       return;
     }
 
     // Calculate and print the results.
     bool succeeded = true;
@@ skipping 29 matching lines @@
 };
 }  // namespace
 }  // namespace benchmark
 
 MojoResult MojoMain(MojoHandle application_request) {
   mojo::ApplicationRunnerChromium runner(new benchmark::BenchmarkApp);
   auto ret = runner.Run(application_request);
   fflush(nullptr);
   return ret;
 }