DigitalGoods: Add fields to ItemDetails mojo to support subsciptions.

Blink impl: crrev.com/c/2497393
Explainer:
https://github.com/WICG/digital-goods/blob/master/explainer.md

Bug: 1140345
Change-Id: Idc6cfd23abcdf1ce696ed26d54804912304108bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2486525
Commit-Queue: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821016}

third_party/blink/public/mojom/digital_goods/digital_goods.mojom

@@ -5,6 +5,7 @@
 module payments.mojom;
 
 import "components/payments/mojom/payment_request_data.mojom";
+import "mojo/public/mojom/base/time.mojom";
 
 // https://github.com/WICG/digital-goods/blob/master/explainer.md
 
@@ -49,6 +50,12 @@ struct ItemDetails {
   string title;
   string description;
   PaymentCurrencyAmount price;
+  // Periods are specified as ISO 8601 durations.
+  // https://en.wikipedia.org/wiki/ISO_8601#Durations
+  string? subscription_period;
+  string? free_trial_period;
+  PaymentCurrencyAmount? introductory_price;
+  string? introductory_price_period;
 };
 
 enum CreateDigitalGoodsResponseCode {