Credential Manager: Add type-checking methods to Web*Credential.

We need to be able to determine what type of credential a given
WebCredential object thinks that it is in a few Chromium-side patches.
This seems like a good way of exposing that information.

BUG=400674

Review URL: https://codereview.chromium.org/618803004

git-svn-id: svn://svn.chromium.org/blink/trunk@183120 bbb929c8-8fbe-4397-9dbb-9b2b20218538