cc: Use HighResNow as timebase if it is fast and reliable

cc/scheduler/delay_based_time_source.h

 // Copyright 2011 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_DELAY_BASED_TIME_SOURCE_H_
 #define CC_SCHEDULER_DELAY_BASED_TIME_SOURCE_H_
 
 #include "base/memory/weak_ptr.h"
 #include "cc/base/cc_export.h"
 #include "cc/scheduler/time_source.h"
 
 namespace base { class SingleThreadTaskRunner; }
 
 namespace cc {
 
 // This timer implements a time source that achieves the specified interval
 // in face of millisecond-precision delayed callbacks and random queueing
-// delays.
+// delays. DelayBasedTimeSource uses base::TimeTicks::Now as its timebase.
 class CC_EXPORT DelayBasedTimeSource : public TimeSource {
  public:
   static scoped_refptr<DelayBasedTimeSource> Create(
       base::TimeDelta interval, base::SingleThreadTaskRunner* task_runner);
 
   virtual void SetClient(TimeSourceClient* client) OVERRIDE;
 
   // TimeSource implementation
   virtual void SetTimebaseAndInterval(base::TimeTicks timebase,
                                       base::TimeDelta interval) OVERRIDE;
@@ skipping 37 matching lines @@
 
   bool active_;
 
   base::SingleThreadTaskRunner* task_runner_;
   base::WeakPtrFactory<DelayBasedTimeSource> weak_factory_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(DelayBasedTimeSource);
 };
 
+// DelayBasedTimeSource uses base::TimeTicks::HighResNow as its timebase.
+class DelayBasedTimeSourceHighRes : public DelayBasedTimeSource {
+ public:
+  static scoped_refptr<DelayBasedTimeSourceHighRes> Create(
+        base::TimeDelta interval, base::SingleThreadTaskRunner* task_runner);
+
+  virtual base::TimeTicks Now() const OVERRIDE;
+
+ protected:
+  DelayBasedTimeSourceHighRes(base::TimeDelta interval,
+                              base::SingleThreadTaskRunner* task_runner);
+  virtual ~DelayBasedTimeSourceHighRes();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(DelayBasedTimeSourceHighRes);
+};
+
 }  // namespace cc
 
 #endif  // CC_SCHEDULER_DELAY_BASED_TIME_SOURCE_H_