title: iOS Admob how to check whether the ad has been loaded
body: <p>I have a banner at the bottom of my modal, and I need to adjust the size of my image accordingly in viewDidLoad method. if the ad is loaded, I need to scale my image down. otherwise I scale it up. The _bannerView is setup 3 seconds before presenting the view. when viewDidLoad. if the ad has not been loaded, I will not display the banner at all for that view. </p>
  
  <p>I don't want to modify the size after viewDidLoad. i.e. I don't want to use admob's delegate method and adjust the size AFTER viewDidLoad. </p>
  
  <p>I have checked GADBannerView but I didn't find any property or method to check. please help</p>