In-app purchase delegate protocol for default purchase handling. The delegate must deliver
the purchased item then call the GADDefaultInAppPurchase object’s finishTransaction method.
Called when the user successfully paid for a purchase. You must first deliver the purchased
item to the user, then call defaultInAppPurchase’s finishTransaction method.
Called when the user clicks on the buy button of an in-app purchase ad. Return YES if the
default purchase flow should be started to purchase the item, otherwise return NO. If not
implemented, defaults to YES.