DOMWindow::navigator should return navigator w/o frame when detached.

Source/modules/vr/NavigatorVRDevice.cpp

Index: Source/modules/vr/NavigatorVRDevice.cpp
diff --git a/Source/modules/vr/NavigatorVRDevice.cpp b/Source/modules/vr/NavigatorVRDevice.cpp
index 6987356e7c603b5b6e4eeb43ba0becf64bef8fa5..eba8cee403dac67f827c061fdf7cbf129c622b1a 100644
--- a/Source/modules/vr/NavigatorVRDevice.cpp
+++ b/Source/modules/vr/NavigatorVRDevice.cpp
@@ -26,9 +26,11 @@ namespace blink {
 NavigatorVRDevice* NavigatorVRDevice::from(Document& document)
 {
     if (!document.frame() || !document.frame()->domWindow())
-        return 0;
-    Navigator& navigator = *document.frame()->domWindow()->navigator();
-    return &from(navigator);
+        return nullptr;
+    Navigator* navigator = document.frame()->domWindow()->navigator();
+    if (!navigator)
+        return nullptr;
+    return &from(*navigator);
 }
 
 NavigatorVRDevice& NavigatorVRDevice::from(Navigator& navigator)