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

cc/scheduler/delay_based_time_source.h

Index: cc/scheduler/delay_based_time_source.h
diff --git a/cc/scheduler/delay_based_time_source.h b/cc/scheduler/delay_based_time_source.h
index 55aac5a97fb5b0ae930aa56ccf80568a8e7248f7..ddb89da3566e19c5ad23dced1a1abcf6e73f019e 100644
--- a/cc/scheduler/delay_based_time_source.h
+++ b/cc/scheduler/delay_based_time_source.h
@@ -15,7 +15,7 @@ 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(
@@ -73,6 +73,23 @@ class CC_EXPORT DelayBasedTimeSource : public TimeSource {
   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_