PaymentApp: Add a interface to query all payment app manifests.
This CL is adding GetAllManifests() to query all payment app manifests
in Chrome layer. So, just rename existing PaymentAppContext in
content/browser/payments to PaymentAppContextImpl and then add a new
PaymentAppContext interface in content/public to expose some APIs to
Chrome layer.
This CL doesn't include actual impelementation now and it will be
completed in follow-up CL.
BUG=661608
Review-Url: https://codereview.chromium.org/2575583003
Cr-Commit-Position: refs/heads/master@{#438569}
10 files changed
