PaymentHandler: Implement permission prompt when calling instruments.set

This patch is an initial implementation of the following spec change:
  - https://github.com/w3c/payment-handler/issues/246
  - https://chromium-review.googlesource.com/c/chromium/src/+/533193

This feature is still behind runtime flag.

Bug: 665949
Change-Id: Ied225b89c7aed3a39955e49e9af2e4e3866a92c2
Reviewed-on: https://chromium-review.googlesource.com/914661
Reviewed-by: Jochen Eisinger <[email protected]>
Reviewed-by: Raymes Khoury <[email protected]>
Reviewed-by: Rouslan Solomakhin <[email protected]>
Reviewed-by: Kinuko Yasuda <[email protected]>
Commit-Queue: Jinho Bang <[email protected]>
Cr-Commit-Position: refs/heads/master@{#539499}
diff --git a/chrome/browser/permissions/permission_request.h b/chrome/browser/permissions/permission_request.h
index b093c8d..61522a7 100644
--- a/chrome/browser/permissions/permission_request.h
+++ b/chrome/browser/permissions/permission_request.h
@@ -44,6 +44,7 @@
   PERMISSION_ACCESSIBILITY_EVENTS = 15,
   PERMISSION_CLIPBOARD_READ = 16,
   PERMISSION_SECURITY_KEY_ATTESTATION = 17,
+  PERMISSION_PAYMENT_HANDLER = 18,
   // NUM must be the last value in the enum.
   NUM
 };