PaymentApp: Implement GetAllManifests() in PaymentAppContext.

This CL is implementing GetAllManifests() method in PaymentAppContext. It is
used to query all manifests data in Chrome layer to display all registered
payment apps in the payment request UI.

Wrote a test for this CL in payment_app_context_impl_unittest.cc and moved
a lot of existing codes in payment_app_manager_unittest.cc to
payment_app_content_unittest_base.cc to share many codes between the new test
and the existing test.

BUG=661608
TEST=payment_app_context_impl_unittest.cc

Review-Url: https://codereview.chromium.org/2556433002
Cr-Commit-Position: refs/heads/master@{#439546}
11 files changed