mac: ensure ui::Compositor exists for visible RWHVs

ui/accelerated_widget_mac/window_resize_helper_mac.cc

Index: ui/accelerated_widget_mac/window_resize_helper_mac.cc
diff --git a/ui/accelerated_widget_mac/window_resize_helper_mac.cc b/ui/accelerated_widget_mac/window_resize_helper_mac.cc
index a3b61836abe1e0c149915897dcfff0b655c78fb0..b64e09b01d1f6cff4fe155e37a7b0c45ad40115f 100644
--- a/ui/accelerated_widget_mac/window_resize_helper_mac.cc
+++ b/ui/accelerated_widget_mac/window_resize_helper_mac.cc
@@ -301,6 +301,10 @@ void WindowResizeHelperMac::Init(
       base::Bind(&WindowResizeHelperMac::EventTimedWait), target_task_runner);
 }
 
+void WindowResizeHelperMac::ShutdownForTests() {
+  task_runner_ = nullptr;
+}
+
 bool WindowResizeHelperMac::WaitForSingleTaskToRun(
     const base::TimeDelta& max_delay) {
   PumpableTaskRunner* pumpable_task_runner =