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

cc/scheduler/begin_frame_tracker.h

Index: cc/scheduler/begin_frame_tracker.h
diff --git a/cc/scheduler/begin_frame_tracker.h b/cc/scheduler/begin_frame_tracker.h
new file mode 100644
index 0000000000000000000000000000000000000000..e903a02b783340f7acf9a6d0993cbd5789f85223
--- /dev/null
+++ b/cc/scheduler/begin_frame_tracker.h
@@ -0,0 +1,97 @@
+// 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/location.h"
+#include "base/trace_event/trace_event.h"
+#include "base/trace_event/trace_event_argument.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 (BFA)
+// usage and provide a few helper methods.
+//
+// With DCHECKs enable, this class checks the following "invariants";
+//  * BFA are monotonically increasing.
+//  * BFA is valid.
+//  * The BFA is only used inside a given period (

[brianderson, 2015/05/01 02:05:21]

Incomplete comment?

[mithro-old, 2015/05/01 03:27:56]

Done.

+//  * A new BFA isn't used before the last BFA is finished with.
+//
+// With the tracing category "cc.debug.scheduler.frames" enabled the tracker
+// will output the following trace information;
+//  * Time period for which the BFA is in usage.
+//  * The flow of BFA as they are passed between tracking objects.
+//
+// TODO(mithro): Record stats about the BeginFrameArgs
+class CC_EXPORT BeginFrameTracker {
+ public:
+  explicit BeginFrameTracker(const tracked_objects::Location& location);
+  ~BeginFrameTracker();
+
+  // The Start and Finish methods manage the period that a BFA should be
+  // accessed for. This allows tight control over the BFA and prevents
+  // accidental usage in the wrong period when code is split across multiple
+  // locations.
+
+  // Start using a new BFA value and check invariant properties.
+  // **Must** only be called after finishing with any previous BFA.
+  void Start(BeginFrameArgs new_args);
+  // Finish using the current BFA.
+  // **Must** only be called while still using a BFA.
+  void Finish();
+
+  // The two accessors methods allow access to the BFA stored inside the
+  // tracker. They are mutually exclusive, at any time it is only valid to call
+  // one or the other. This makes sure you understand exactly which BFA you are
+  // intending to use and verifies that is the case.
+
+  // Get the current BFA object.
+  // **Must** only be called between the start and finish methods calls.
+  const BeginFrameArgs& Current() const;
+  // Get the last used BFA.
+  // **Must** only be called when **not** between the start and finish method
+  // calls.
+  const BeginFrameArgs& Last() const;
+
+  // Helper method to try and return a valid interval property. Defaults to
+  // BFA::DefaultInterval() is no other interval can be found. Can be called at
+  // any time.
+  base::TimeDelta Interval() const;
+
+  void AsValueInto(base::TimeTicks now,
+                   base::trace_event::TracedValue* dict) const;
+
+  // The following methods violate principles of how BeginFrameArgs should be
+  // used. These methods should only be used when there is no other choice.
+  bool DangerousMethodHasStarted() const {
+    return !current_updated_at_.is_null();
+  }
+  bool DangerousMethodHasFinished() const { return HasFinished(); }
+  const BeginFrameArgs& DangerousMethodCurrentOrLast() const;
+
+ private:
+  // Return if currently not between the start/end period. This method should
+  // be used extremely sparingly and normal indicates incorrect management of
+  // the BFA object. Can be called at any time.
+  bool HasFinished() const { return !current_finished_at_.is_null(); }
+
+  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_