cc:: Change plumbing for external_begin_frame_source to the Scheduler.

cc/debug/micro_benchmark_controller_unittest.cc

Index: cc/debug/micro_benchmark_controller_unittest.cc
diff --git a/cc/debug/micro_benchmark_controller_unittest.cc b/cc/debug/micro_benchmark_controller_unittest.cc
index 6494c86660925fc2da2b023548388c5e75793f7d..a66d2dcb13d08acdf5883ccf2b8cc79d72193a9f 100644
--- a/cc/debug/micro_benchmark_controller_unittest.cc
+++ b/cc/debug/micro_benchmark_controller_unittest.cc
@@ -35,7 +35,7 @@ class MicroBenchmarkControllerTest : public testing::Test {
     layer_tree_host_->SetRootLayer(Layer::Create(LayerSettings()));
     layer_tree_host_->InitializeForTesting(
         TaskRunnerProvider::Create(nullptr, nullptr),
-        scoped_ptr<Proxy>(new FakeProxy));
+        scoped_ptr<Proxy>(new FakeProxy), nullptr);
   }
 
   void TearDown() override {