Credential Manager: Wire up the client.

Now that we've added a mock CredentialManagerClient to content shell, we
can wire up the various CredentialsContainer methods to the client
provided by the embedder.

BUG=400674

Review URL: https://codereview.chromium.org/573663002

git-svn-id: svn://svn.chromium.org/blink/trunk@182052 bbb929c8-8fbe-4397-9dbb-9b2b20218538