Fix crash when closing a constrained window in win aura. The crash happens...

Fix crash when closing a constrained window in win aura. The crash happens because the aura::Window for the constrained window is removing itself from its parent window, and its parent_ pointer is already NULL'd before the focus manager is retrieved which fails because aura::client::GetFocusClient can't get to a root window. See the bug for the callstack.

BUG=174727
Review URL: https://codereview.chromium.org/12226102

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@181947 0039d316-1c4b-4281-b951-d872f2087c98

ui/views/widget/native_widget_aura.cc

@@ -859,8 +859,9 @@ void NativeWidgetAura::OnWindowFocused(aura::Window* gained_focus,
       DCHECK_EQ(ownership_, Widget::InitParams::WIDGET_OWNS_NATIVE_WIDGET);
     }
 
-    delegate_->OnNativeBlur(
-        aura::client::GetFocusClient(window_)->GetFocusedWindow());
+    aura::client::FocusClient* client = aura::client::GetFocusClient(window_);
+    if (client)  // NULL during destruction of aura::Window.
+      delegate_->OnNativeBlur(client->GetFocusedWindow());
   }
 }