Trace main thread commit in flow events.

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 9d1c36657e1e410aec6c624f28fe77fef521040d..1b1616c35c0828ca2c6bced908baaad03e8fb792 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -367,6 +367,7 @@ void LayerTreeHost::FinishCommitOnImplThread(LayerTreeHostImpl* host_impl) {
 }
 
 void LayerTreeHost::WillCommit() {
+  OnCommitForSwapPromises();
   client_->WillCommit();
 }
 
@@ -1270,6 +1271,11 @@ void LayerTreeHost::BreakSwapPromises(SwapPromise::DidNotSwapReason reason) {
   swap_promise_list_.clear();
 }
 
+void LayerTreeHost::OnCommitForSwapPromises() {
+  for (auto* swap_promise : swap_promise_list_)
+    swap_promise->OnCommit();
+}
+
 void LayerTreeHost::set_surface_id_namespace(uint32_t id_namespace) {
   surface_id_namespace_ = id_namespace;
 }