Credential Manager: Correctly return LocalCredential or FederatedCredential objects.

Currently, we're hard-coding a return type of LocalCredential in
'.request()'s '::onSuccess' handler. This patch adjusts that now that we
can distinguish between local and federated WebCredential objects.

R=vabr@chromium.org
BUG=400674

Review URL: https://codereview.chromium.org/635773002

git-svn-id: svn://svn.chromium.org/blink/trunk@183322 bbb929c8-8fbe-4397-9dbb-9b2b20218538