Add granular file tracing.

base/trace_event/scoped_file_trace.h

Index: base/trace_event/scoped_file_trace.h
diff --git a/base/trace_event/scoped_file_trace.h b/base/trace_event/scoped_file_trace.h
new file mode 100644
index 0000000000000000000000000000000000000000..61fa256ad37e81b4ec7687d8725ed18fe71a1b17
--- /dev/null
+++ b/base/trace_event/scoped_file_trace.h
@@ -0,0 +1,51 @@
+// 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.
+
+#ifndef BASE_TRACE_EVENT_SCOPED_FILE_TRACE_H_
+#define BASE_TRACE_EVENT_SCOPED_FILE_TRACE_H_
+
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/macros.h"
+
+namespace base {
+
+class FilePath;
+
+// A class to trace file I/O. Just stick one in a scope you're tracing like so:
+//
+//   FilePath path("/var/run/prog");
+//   File prog(path, File::FILE_OPEN | File::FILE_WRITE);
+//   {
+//     ScopedFileTrace trace(path, "Write", 1);
+//     prog.Write(0, "1", 1);
+//   }
+//
+// Tracing will be started/stopped automatically based on the scope it lives in.
+//
+class ScopedFileTrace {
+ public:
+  ScopedFileTrace(const FilePath& path, const char* event, size_t bytes);
+  ~ScopedFileTrace();
+
+  // The group that shows up in chrome://tracing.
+  static const char kGroup[];

[Dan Beam, 2015/04/23 22:32:19]

don't really care where this lives; it's arguable it should be reversed
(base::File::kTraceGroup and it's used here).

+
+ private:
+  // The file path being traced.
+  std::string path_;

[Dan Beam, 2015/04/24 03:45:18]

Note to self: const

+
+  // The event to trace (e.g. "Read", "Write").
+  const char* event_;
+
+  // Number of bytes involved in this event. Not reported if 0.
+  size_t bytes_;

[Dan Beam, 2015/04/24 03:45:18]

Also const if it's too hard to update with real byte amount later

+
+  DISALLOW_COPY_AND_ASSIGN(ScopedFileTrace);
+};
+
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_SCOPED_FILE_TRACE_H_