PaymentRequest should be a ContextLifecycleObserver

This CL makes PaymentRequest a ContextLifecycleObserver.
Since mojo::Binding::Close() has a precondition that it
is bound, check that in PaymentRequest::cleanUp before
calling Close(), if not Payment unit tests that create
PaymentRequests that do not bind will crash.

BUG=602660

Review URL: https://codereview.chromium.org/1880353004

Cr-Commit-Position: refs/heads/master@{#387214}
2 files changed