Hoist begin frame sources out of scheduler

cc/test/layer_tree_test.cc

Index: cc/test/layer_tree_test.cc
diff --git a/cc/test/layer_tree_test.cc b/cc/test/layer_tree_test.cc
index 98396eaee38c065db0dea884d3abab2166b99e1f..d93b0bd100a21751c07825bc532ce94e8f8bec56 100644
--- a/cc/test/layer_tree_test.cc
+++ b/cc/test/layer_tree_test.cc
@@ -955,7 +955,8 @@ scoped_ptr<OutputSurface> LayerTreeTest::CreateOutputSurface() {
             output_surface->capabilities().delegated_rendering);
   output_surface_ = output_surface.get();
 
-  if (settings_.use_external_begin_frame_source) {
+  if (settings_.use_external_begin_frame_source &&
+      settings_.wait_for_beginframe_interval) {
     DCHECK(external_begin_frame_source_);
     DCHECK(external_begin_frame_source_->is_ready());
   }