Bug 494715: When panning the content and switching tabs, viewport can be wrong, r=combee

mobile/chrome/content/WidgetStack.js

@@ -768,6 +768,9 @@ WidgetStack.prototype = {
     if (this._skipViewportUpdates)
       return
 
+    if (this._forceViewportUpdate)
+      this._viewportUpdate();
+
     let boundsSizeChanged =
       this._startViewportBoundsString != this._viewportBounds.toString();
     this._callViewportUpdateHandler(boundsSizeChanged || this._forceViewportUpdate);