cc: Adding BeginFrameTracker object and removing Now() from LTHI.

cc/scheduler/begin_frame_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 CC_SCHEDULER_BEGIN_FRAME_TRACKER_H_
+#define CC_SCHEDULER_BEGIN_FRAME_TRACKER_H_
+
+#include <set>
+#include <string>
+
+#include "base/debug/trace_event.h"
+#include "base/location.h"
+#include "cc/output/begin_frame_args.h"
+
+#define BEGINFRAMETRACKER_FROM_HERE FROM_HERE_WITH_EXPLICIT_FUNCTION("")
+
+namespace cc {
+
+// Microclass to trace and check properties for correct BeginFrameArgs usage.

[brianderson, 2014/12/18 02:01:38]

Can you explain more about why this class exists? It looks like it helps
centralize some of the uninitialized interval logic. Does it help centralize
anything else?

[mithro-old, 2014/12/18 17:22:40]

It centralises a bunch of things;
 * State about the BeginFrameArgs - If they are currently "in use" or node.

 * A large amount of sanity checks that both make sure that the incoming
BeginFrameArgs are sane and that the usage of them is sane. This is how I
discovered not having a valid BeginFrameArgs on the first commit problem.

 * A large amount of debugging code for helping trace the BeginFrameArgs through
the system. When you enable the tracing you get arrows which show how the
BeginFrameArgs flow between tasks. This is pretty awesome to see.

In the future, I can potentially see;

 * UMA tracking and other stats recording about the BeginFrameArgs objects.

 * More helper methods along the same vein as the Interval() one (not quite sure
what they would be yet).

 * Better auditing of BeginFrameArgs usage.

+// TODO(mithro): Record stats about the BeginFrameArgs
+class CC_EXPORT BeginFrameTracker {

[brianderson, 2014/12/18 02:01:38]

Can you provide descriptions for each of the public methods in this class?

[mithro-old, 2014/12/18 17:22:40]

Done.

+ public:
+  explicit BeginFrameTracker(const tracked_objects::Location& location);
+  ~BeginFrameTracker();
+
+  void Start(BeginFrameArgs new_args);
+  const BeginFrameArgs& Get() const;
+  bool HasFinished() const { return !current_finished_at_.is_null(); }
+  void Finish();
+  const BeginFrameArgs& Last() const;
+
+  base::TimeDelta Interval() const;
+
+ private:
+  const tracked_objects::Location location_;
+  const std::string location_string_;
+
+  base::TimeTicks current_updated_at_;
+  BeginFrameArgs current_args_;
+  base::TimeTicks current_finished_at_;
+};
+
+}  // namespace cc
+
+#endif  // CC_SCHEDULER_BEGIN_FRAME_TRACKER_H_