Issue #1643 - Follow up: Add a null check for mOwner in ResizeObserverNotificationHelper::Unregister

A race condition seemed to exist between tab destruction and un-registering a ResizeObserver resulting in a null deref crash.
The original reporter in Forum Topic 25311 experienced this on msn.com so that was the functional test reference.

dom/base/ResizeObserverController.cpp

@@ -58,6 +58,12 @@ ResizeObserverNotificationHelper::Register()
 void
 ResizeObserverNotificationHelper::Unregister()
 {
+  if (!mOwner) {
+    // We've outlived our owner, so there's nothing registered anymore.
+    mRegistered = false;
+    return;
+  }
+
   if (!mRegistered) {
     return;
   }