Fix Credential Management API Store() for existing Credentials

This changes fixes the Credential Management API to correctly handle
storing of already existing credentials. In the previous version
`preferred_match()` was updated, which is not necessarily the credential
selected by the user.

Bug: 795878
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I269f465861f44cdd784f0ce077e755191d3bd7bd
Reviewed-on: https://chromium-review.googlesource.com/843022
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Maxim Kolosovskiy <kolos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526313}
GURL CredentialManagerImpl::GetLastCommittedURL() const {
  return client_->GetLastCommittedEntryURL();
}

GURL CredentialManagerImpl::GetLastCommittedURL() const {
  return client_->GetLastCommittedEntryURL();
}
