Bug 958254 - Remove an unnecessary (and harmful) assertion. r=ajones

dom/ipc/TabChild.cpp

@@ -590,7 +590,6 @@ TabChild::HandlePossibleViewportChange()
   // by AsyncPanZoomController and causes a blurry flash.
   bool isFirstPaint;
   nsresult rv = utils->GetIsFirstPaint(&isFirstPaint);
-  MOZ_ASSERT(NS_SUCCEEDED(rv));
   if (NS_FAILED(rv) || isFirstPaint) {
     // FIXME/bug 799585(?): GetViewportInfo() returns a defaultZoom of
     // 0.0 to mean "did not calculate a zoom".  In that case, we default