Fixes to WindowTreeHostMus

ui/aura/test/mus/test_window_tree.cc

Index: ui/aura/test/mus/test_window_tree.cc
diff --git a/ui/aura/test/mus/test_window_tree.cc b/ui/aura/test/mus/test_window_tree.cc
index 1a437040965ca6075d5e86ca5279df7ca2ad36c6..2487007bd4bba9fd1705cdc6f1fc80623ad583aa 100644
--- a/ui/aura/test/mus/test_window_tree.cc
+++ b/ui/aura/test/mus/test_window_tree.cc
@@ -85,6 +85,15 @@ bool TestWindowTree::GetAndRemoveFirstChangeOfType(WindowTreeChangeType type,
   return false;
 }
 
+size_t TestWindowTree::GetChangeCountForType(WindowTreeChangeType type) {
+  size_t count = 0;
+  for (const auto& change : changes_) {
+    if (change.type == type)
+      ++count;
+  }
+  return count;
+}
+
 void TestWindowTree::OnChangeReceived(uint32_t change_id,
                                       WindowTreeChangeType type) {
   changes_.push_back({type, change_id});