Adding AsValue and TypeString methods to DelayBasedTimeSource.

cc/scheduler/time_source.h

Index: cc/scheduler/time_source.h
diff --git a/cc/scheduler/time_source.h b/cc/scheduler/time_source.h
index 3b8d048886511c9b5b0232035183950df2d31375..ba2fce071858018d4d825e95c486d23146b0f619 100644
--- a/cc/scheduler/time_source.h
+++ b/cc/scheduler/time_source.h
@@ -36,8 +36,8 @@ class CC_EXPORT TimeSource : public base::RefCounted<TimeSource> {
   virtual bool Active() const = 0;
   virtual void SetTimebaseAndInterval(base::TimeTicks timebase,
                                       base::TimeDelta interval) = 0;
-  virtual base::TimeTicks LastTickTime() = 0;
-  virtual base::TimeTicks NextTickTime() = 0;
+  virtual base::TimeTicks LastTickTime() const = 0;
+  virtual base::TimeTicks NextTickTime() const = 0;
 
  protected:
   virtual ~TimeSource() {}