[Payment Handler] Don't wait for response from closed payment app.

Before this patch, tapping the back button on top of the payment handler
window on desktop would not affect the |response_helper_|, which would
continue waiting for a response from the payment app. The service worker
of the closed payment app could timeout after 5 minutes and invoke the
|response_helper_|. Depending on what else the user did afterwards, in
the best case scenario, the payment sheet would display a "Transaction
failed" error message. In the worst case scenario, the
|response_helper_| would be used after free.

This patch clears the |response_helper_| in the PaymentRequestState and
in the ServiceWorkerPaymentInstrument after the payment app is closed.

After this patch, the cancelled payment app does not show "Transaction
failed" and does not use memory after it was freed.

Bug: 956597
Change-Id: I64134b911a4f8c154cb56d537a8243a68a806394
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1588682
Reviewed-by: anthonyvd <anthonyvd@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654995}
bool PaymentRequestState::IsInitialized() const {
  return get_all_instruments_finished_;
}

bool PaymentRequestState::IsInitialized() const {
  return get_all_instruments_finished_;
}
