Add SwapPromise support to LayerTreeHost and LayerTreeImpl

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 d70d7971db0819bff77f795a91934688dc4244a2..de0b9076c77374e2018a7859872b5dc798170d2f 100644
--- a/cc/test/layer_tree_test.cc
+++ b/cc/test/layer_tree_test.cc
@@ -409,6 +409,13 @@ void LayerTreeTest::PostSetNeedsCommitToMainThread() {
                                          main_thread_weak_ptr_));
 }
 
+void LayerTreeTest::PostSetNeedsUpdateLayersToMainThread() {
+  main_task_runner_->PostTask(
+      FROM_HERE,
+      base::Bind(&LayerTreeTest::DispatchSetNeedsUpdateLayers,
+                 main_thread_weak_ptr_));
+}
+
 void LayerTreeTest::PostReadbackToMainThread() {
   main_task_runner_->PostTask(
       FROM_HERE,
@@ -538,6 +545,13 @@ void LayerTreeTest::DispatchSetNeedsCommit() {
     layer_tree_host_->SetNeedsCommit();
 }
 
+void LayerTreeTest::DispatchSetNeedsUpdateLayers() {
+  DCHECK(!proxy() || proxy()->IsMainThread());
+
+  if (layer_tree_host_)
+    layer_tree_host_->SetNeedsUpdateLayers();
+}
+
 void LayerTreeTest::DispatchReadback() {
   DCHECK(!proxy() || proxy()->IsMainThread());