Mozart: Correctly unregister views.

We need to tell ViewStub that we're releasing the view before we
actually unregister.  This was caught by a safety check in ViewStub's
destructor to ensure everything the ViewStub owns is cleaned up
properly before the ViewStub itself is dropped on the floor.

BUG=
R=abarth@google.com

Review URL: https://codereview.chromium.org/1759003002 .

services/ui/view_manager/view_registry.cc

diff --git a/services/ui/view_manager/view_registry.cc b/services/ui/view_manager/view_registry.cc
index 410537e..4868d0d 100644
--- a/services/ui/view_manager/view_registry.cc
+++ b/services/ui/view_manager/view_registry.cc
@@ -459,8 +459,9 @@
 void ViewRegistry::UnregisterViewStub(std::unique_ptr<ViewStub> view_stub) {
   DCHECK(view_stub);
 
-  if (view_stub->state())
-    UnregisterView(view_stub->state());
+  ViewState* view_state = view_stub->ReleaseView();
+  if (view_state)
+    UnregisterView(view_state);
 }
 
 void ViewRegistry::SetLayout(ViewStub* view_stub,