Turn InAppPurchase into an EventEmitter

atom/browser/mac/in_app_purchase.h

@@ -22,7 +22,7 @@ bool CanMakePayments(void);
 std::string GetReceiptURL(void);
 
 void PurchaseProduct(const std::string& productID,
-                     const int quantity,
+                     int quantity,
                      const InAppPurchaseCallback& callback);
 
 }  // namespace in_app_purchase

atom/browser/mac/in_app_purchase.mm

@@ -146,9 +146,9 @@ std::string GetReceiptURL() {
 }
 
 void PurchaseProduct(const std::string& productID,
-                     const int quantity,
+                     int quantity,
                      const InAppPurchaseCallback& callback) {
-  auto iap =
+  auto* iap =
       [[InAppPurchase alloc] initWithCallback:callback quantity:quantity];
 
   [iap purchaseProduct:base::SysUTF8ToNSString(productID)];

atom/browser/mac/in_app_purchase_observer.mm

@@ -177,6 +177,8 @@
 namespace in_app_purchase {
 
 void AddTransactionObserver(const InAppTransactionCallback& callback) {
+  // This is leaked, but we should be fine since we don't have a way to remove
+  // callback and the inAppPurchase module is never unloaded.
   [[InAppTransactionObserver alloc] initWithCallback:callback];
 }