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Write PaymentAppDatabase class and then just move most of logics to access
database from PaymentAppManager to the new class.

The class is providing APIs to read/write payment app related data(e.g. manifest
and associated service worker). It can be shared between payment_app_manager.cc
and payment_app_context.cc.

The payment_app_manager.cc contains a actual implementation of the
PaymentAppManager interface[1] used on the page or worker.
Also, the payment_app_context.cc can be used to query all manifests data.
in order to update payment request UI in Chrome layer.

[1] https://w3c.github.io/webpayments-payment-apps-api/#payment-app-manager

BUG=661608

Review-Url: https://codereview.chromium.org/2572183002
Cr-Commit-Position: refs/heads/master@{#439100}