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

cc/test/fake_layer_tree_host_impl.cc

Index: cc/test/fake_layer_tree_host_impl.cc
diff --git a/cc/test/fake_layer_tree_host_impl.cc b/cc/test/fake_layer_tree_host_impl.cc
index bfac501fe0d743dade4d47ad55fc40d36493508c..5ca82dbf93f4b5d497b9ce4ae613569776b776ea 100644
--- a/cc/test/fake_layer_tree_host_impl.cc
+++ b/cc/test/fake_layer_tree_host_impl.cc
@@ -57,14 +57,19 @@ void FakeLayerTreeHostImpl::CreatePendingTree() {
 }
 
 BeginFrameArgs FakeLayerTreeHostImpl::CurrentBeginFrameArgs() const {
-  if (!current_begin_frame_args_.IsValid())
-    return LayerTreeHostImpl::CurrentBeginFrameArgs();
-  return current_begin_frame_args_;
+  if (!current_begin_frame_args_.HasFinished()) {
+    return current_begin_frame_args_.Get();
+  } else {
+    return current_begin_frame_args_.Last();
+  }
 }
 
 void FakeLayerTreeHostImpl::SetCurrentBeginFrameArgs(
     const BeginFrameArgs& args) {
-  current_begin_frame_args_ = args;
+  if (!current_begin_frame_args_.HasFinished()) {
+    current_begin_frame_args_.Finish();
+  }
+  current_begin_frame_args_.Start(args);
 }
 
 int FakeLayerTreeHostImpl::RecursiveUpdateNumChildren(LayerImpl* layer) {