[WebPayments] Fixing segfault which occurs after spec update

Bug: 749815
Change-Id: Ife1a72146c3f75020286fb588f5e78a12e8d747a
Reviewed-on: https://chromium-review.googlesource.com/596412Reviewed-by: Anthony Vallee-Dubois <anthonyvd@chromium.org>
Commit-Queue: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491366}

chrome/browser/ui/views/payments/payment_request_item_list.cc

@@ -187,6 +187,7 @@ void PaymentRequestItemList::AddItem(
 
 void PaymentRequestItemList::Clear() {
   items_.clear();
+  selected_item_ = nullptr;
 }
 
 std::unique_ptr<views::View> PaymentRequestItemList::CreateListView() {