Update from https://crrev.com/303153

base/profiler/scoped_tracker.h

 // Copyright 2014 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_PROFILER_SCOPED_TRACKER_H_
 #define BASE_PROFILER_SCOPED_TRACKER_H_
 
 //------------------------------------------------------------------------------
 // Utilities for temporarily instrumenting code to dig into issues that were
 // found using profiler data.
 
 #include "base/base_export.h"
 #include "base/callback_forward.h"
 #include "base/location.h"
 #include "base/profiler/scoped_profile.h"
 
 namespace tracked_objects {
 
 // ScopedTracker instruments a region within the code if the instrumentation is
 // enabled. It can be used, for example, to find out if a source of jankiness is
 // inside the instrumented code region.
+// Details:
+// 1. This class creates a task (like ones created by PostTask calls or IPC
+// message handlers). This task can be seen in chrome://profiler and is sent as
+// a part of profiler data to the UMA server. See profiler_event.proto.
+// 2. That task's lifetime is same as the lifetime of the ScopedTracker
+// instance.
+// 3. The execution time associated with the task is the wallclock time between
+// its constructor and destructor, minus wallclock times of directly nested
+// tasks.
+// 4. Task creation that this class utilizes is highly optimized.
+// 5. The class doesn't create a task unless this was enabled for the current
+// process. Search for ScopedTracker::Enable for the current list of processes
+// and channels where it's activated.
+// 6. The class is designed for temporarily instrumenting code to find
+// performance problems, after which the instrumentation must be removed.
 class BASE_EXPORT ScopedTracker {
  public:
   ScopedTracker(const Location& location);
 
   // Enables instrumentation for the remainder of the current process' life. If
   // this function is not called, all profiler instrumentations are no-ops.
   static void Enable();
 
   // Augments a |callback| with provided |location|. This is useful for
   // instrumenting cases when we know that a jank is in a callback and there are
   // many possible callbacks, but they come from a relatively small number of
   // places. We can instrument these few places and at least know which one
   // passes the janky callback.
   static base::Closure TrackCallback(const Location& location,
                                      const base::Closure& callback);
 
  private:
   const ScopedProfile scoped_profile_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedTracker);
 };
 
 }  // namespace tracked_objects
 
 #endif  // BASE_PROFILER_SCOPED_TRACKER_H_