Credential Manager: Remove WebString from WebCredentialManagerError.
Credential Manager: Remove WebString from WebCredentialManagerError.

The enum gives enough detail; we don't need an additional string to be
created and passed around.

BUG=400674

Review URL: https://codereview.chromium.org/618723002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@183056 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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