MacViews: Fix WidgetWithDestroyedNativeViewTest.Test

Regressed in r309924. NativeWidgetMac just needs to check for a null
bridge_ to handle calls to {Set,Get}NativeWindowProperty during
asynchronous close calls.

BUG=378134

Review URL: https://codereview.chromium.org/826083003

Cr-Commit-Position: refs/heads/master@{#310730}

ui/views/widget/native_widget_mac.mm

@@ -195,11 +195,15 @@ void NativeWidgetMac::ViewRemoved(View* view) {
 }
 
 void NativeWidgetMac::SetNativeWindowProperty(const char* name, void* value) {
-  bridge_->SetNativeWindowProperty(name, value);
+  if (bridge_)
+    bridge_->SetNativeWindowProperty(name, value);
 }
 
 void* NativeWidgetMac::GetNativeWindowProperty(const char* name) const {
-  return bridge_->GetNativeWindowProperty(name);
+  if (bridge_)
+    return bridge_->GetNativeWindowProperty(name);
+
+  return nullptr;
 }
 
 TooltipManager* NativeWidgetMac::GetTooltipManager() const {