[Startup Tracing] Enable startup tracing from config file

components/tracing/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 "components/tracing/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.h"
+
+namespace tracing {
+
+namespace {
+
+// Trace config file path:
+// - Android: /data/local/.config/chrome-trace-config.json
+// - POSIX other than Android: $HOME/.config/chrome-trace-config.json
+// - Win: %USERPROFILE%/.config/chrome-trace-config.json
+#if defined(OS_ANDROID)
+const base::FilePath::CharType kAndroidTraceConfigDir[] =
+    FILE_PATH_LITERAL("/data/local");
+#endif
+
+const base::FilePath::CharType kChromeConfigDir[] =
+    FILE_PATH_LITERAL(".config");
+const base::FilePath::CharType kTraceConfigFileName[] =
+    FILE_PATH_LITERAL("chrome-trace-config.json");
+
+base::FilePath GetTraceConfigFilePath() {
+#if defined(OS_ANDROID)
+  base::FilePath path(kAndroidTraceConfigDir);
+#elif defined(OS_POSIX) || defined(OS_WIN)
+  base::FilePath path;
+  PathService::Get(base::DIR_HOME, &path);
+#else
+  base::FilePath path;
+#endif
+  path = path.Append(kChromeConfigDir);
+  path = path.Append(kTraceConfigFileName);
+  return path;
+}
+
+} // namespace
+
+void EnableStartupTracingIfConfigFileExists() {
+  base::FilePath trace_config_file_path = GetTraceConfigFilePath();
+  if (trace_config_file_path.empty()

[dsinclair, 2015/06/15 13:49:45]

This can never be empty given the above always appends kChromeConfigDir and
kTraceConfigFileName.

[Zhen Wang, 2015/06/15 16:58:30]

Done.

+      || !base::PathExists(trace_config_file_path))
+    return;
+
+  std::string trace_config_str;
+  if (!base::ReadFileToString(trace_config_file_path, &trace_config_str))

[dsinclair, 2015/06/15 13:49:45]

Should we verify the file size before reading? Could it be possible to put a
huge file here and the browser runs out of memory trying to read it?

[nednguyen, 2015/06/15 15:03:39]

Looks like there is a ReadFileToString with a max size limit
(https://code.google.com/p/chromium/codesearch#chromium/src/base/files/file_ut...)

[Zhen Wang, 2015/06/15 16:58:30]

Done.

+    return;
+
+  base::trace_event::TraceConfig trace_config(trace_config_str);
+  base::trace_event::TraceLog::GetInstance()->SetEnabled(
+      trace_config, base::trace_event::TraceLog::RECORDING_MODE);
+}
+
+}  // namespace tracing