[Startup Tracing] Enable startup tracing from config file

base/trace_event/startup_tracing.cc

+// Copyright (c) 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 "base/trace_event/startup_tracing.h"
+
+#include "base/files/file_path.h"
+#include "base/files/file_util.h"
+#include "base/path_service.h"
+#include "base/trace_event/trace_event_impl.h"
+
+namespace base {
+namespace trace_event {
+
+namespace {
+
+// Trace config file path:
+// - Android: /data/local/.chrome-telemetry/chrome-trace-config.json
+// - POSIX other than Android: $HOME/.chrome-telemetry/chrome-trace-config.json
+// - Win: %USERPROFILE%/.chrome-telemetry/chrome-trace-config.json

[nednguyen, 2015/06/12 20:12:18]

I am not sure whether putting the name "telemetry" here is a good idea.  

I would rather have the chrome browser system to not make any assumption about
the naming of some test automation system that runs it. 

How about just "../chrome-config/chrome-trace-config.json" ?

[Zhen Wang, 2015/06/13 03:48:03]

Now changed to .config, which is the convention.
https://code.google.com/p/chromium/codesearch#chromium/src/chrome/common/chro...

We do not use GetDefaultUserDataDirectory() because Telemetry does not know if
the browser is official build or not.

+#if defined(OS_ANDROID)
+const FilePath::CharType kAndroidTraceConfigDir[] =
+    FILE_PATH_LITERAL("/data/local");
+#endif
+
+const FilePath::CharType kChromeTelemetryDir[] =
+    FILE_PATH_LITERAL(".chrome-telemetry");
+const FilePath::CharType kTraceConfigFileName[] =
+    FILE_PATH_LITERAL("chrome-trace-config.json");
+
+FilePath GetTraceConfigFilePath() {
+#if defined(OS_ANDROID)
+  FilePath path(kAndroidTraceConfigDir);
+#elif defined(OS_POSIX) || defined(OS_WIN)
+  FilePath path;
+  PathService::Get(DIR_HOME, &path);
+#else
+  FilePath path;
+#endif
+  path = path.Append(kChromeTelemetryDir);
+  path = path.Append(kTraceConfigFileName);
+  return path;
+}
+
+} // namespace
+
+void EnableStartupTracingIfConfigFileExists() {
+  FilePath trace_config_file_path = GetTraceConfigFilePath();
+  if (trace_config_file_path.empty() || !PathExists(trace_config_file_path))
+    return;
+
+  std::string trace_config_str;
+  if (!ReadFileToString(trace_config_file_path, &trace_config_str))
+    return;
+
+  TraceConfig trace_config(trace_config_str);
+  TraceLog::GetInstance()->SetEnabled(trace_config, TraceLog::RECORDING_MODE);
+}
+
+}  // namespace trace_event
+}  // namespace base