Update from https://crrev.com/303153

base/profiler/scoped_tracker.h

Index: base/profiler/scoped_tracker.h
diff --git a/base/profiler/scoped_tracker.h b/base/profiler/scoped_tracker.h
index a1885571351a7a7ca31a3d63e80342743b0d220f..f83654e1bd48e55fa7ddd3f88837871dd4eff2c7 100644
--- a/base/profiler/scoped_tracker.h
+++ b/base/profiler/scoped_tracker.h
@@ -19,6 +19,21 @@ 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);