Applies the list of mutate operations (ie. add, set, remove):
Add - Creates a new {@linkplain AdGroupAd ad group ad}. The {@code adGroupId} mustreference an existing ad group. The child {@code Ad} mustbe sufficiently specified by constructing a concrete ad type (such as {@code TextAd}) and setting its fields accordingly.
Set - Updates an ad group ad. Except for {@code status}, ad group ad fields are not mutable. Status updates are straightforward - the status of the ad group ad is updated as specified. If any other field has changed, it will be ignored. If you want to change any of the fields other than status, you must make a new ad and then delete the old one.
Remove - Removes the link between the specified AdGroup and Ad.
@param operations The operations to apply.
@return A list of AdGroupAds where each entry in the list is the resultof applying the operation in the input list with the same index. For an add/set operation, the return AdGroupAd will be what is saved to the db. In the case of the remove operation, the return AdGroupAd will simply be an AdGroupAd containing an Ad with the id set to the Ad being deleted from the AdGroup.