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

cc/test/scheduler_test_common.h

Index: cc/test/scheduler_test_common.h
diff --git a/cc/test/scheduler_test_common.h b/cc/test/scheduler_test_common.h
index 633289ba3ff729cf5f909d0201b2dd0410185f46..a2982ee5fdaedb8ea8faafea30765337bfcd202b 100644
--- a/cc/test/scheduler_test_common.h
+++ b/cc/test/scheduler_test_common.h
@@ -184,7 +184,6 @@ class TestScheduler : public Scheduler {
 
   BeginFrameSource& frame_source() { return *frame_source_; }
   bool FrameProductionThrottled() { return throttle_frame_production_; }
-  BeginFrameArgs begin_impl_frame_args() { return begin_impl_frame_args_; }
 
   ~TestScheduler() override;
 
@@ -195,6 +194,10 @@ class TestScheduler : public Scheduler {
     }
   }
 
+  base::TimeDelta BeginImplFrameInterval() {
+    return begin_impl_frame_tracker_.Interval();
+  }
+
  protected:
   // Overridden from Scheduler.
   base::TimeTicks Now() const override;