`Product` of (`InAppPurchase`) - Miss match between documentation/types to result of the key `downloadable`.


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### Issue Details

* **Electron Version:**
`v5.0.3`

* **Operating System:**
macOS 10.14.15

* **Last Known Working Electron version:**
N/A

### Expected Behavior

The `Product` structure of returned data from `inAppPurchase.getProducts()` should contain `downloadable` key as in the [docs / TS type definitions](https://github.com/electron/electron/blob/master/docs/api/structures/product.md)


### Actual Behavior

The `Product` contains `isDownloadable` key (which isn't documented).


### To Reproduce


### Screenshots

![Screen Shot 2019-06-10 at 11 54 30](https://user-images.githubusercontent.com/2685242/59184392-b58f9b00-8b76-11e9-99a0-58fb7eead403.png)


### Additional Information

I think the issue related to [this line](https://github.com/electron/electron/blob/3e5a98b5f43fe70961eb6452dc8837453c6de1a0/atom/browser/api/atom_api_in_app_purchase.cc#L64)



atom/browser/api/atom_api_in_app_purchase.cc
const in_app_purchase::Product& val) {
    mate::Dictionary dict = mate::Dictionary::CreateEmpty(isolate);
    dict.SetHidden("simple", true);
    dict.Set("productIdentifier", val.productIdentifier);
    dict.Set("localizedDescription", val.localizedDescription);
    dict.Set("localizedTitle", val.localizedTitle);
    dict.Set("contentVersion", val.localizedTitle);
    dict.Set("contentLengths", val.contentLengths);
    
    dict.Set("price", val.price);
    dict.Set("formattedPrice", val.formattedPrice);
    
    dict.Set("isDownloadable", val.downloadable);
    return dict.GetHandle();
  }
};
}  
namespace atom {
namespace api {
#if defined(OS_MACOSX)
mate::Handle<InAppPurchase> InAppPurchase::Create(v8::Isolate* isolate) {