Credential Management: Add 'Credential::type' attribute.

Source/modules/credentialmanager/Credential.idl

Index: Source/modules/credentialmanager/Credential.idl
diff --git a/Source/modules/credentialmanager/Credential.idl b/Source/modules/credentialmanager/Credential.idl
index 76eab5bc97435afff0658ffebbdf5b34077c6ec7..2c3c2a1b531c63069bf13d6a524e19814d8e4819 100644
--- a/Source/modules/credentialmanager/Credential.idl
+++ b/Source/modules/credentialmanager/Credential.idl
@@ -2,11 +2,16 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// https://w3c.github.io/webappsec/specs/credentialmanagement/#credential
+
 [
     RuntimeEnabled=CredentialManager,
     GarbageCollected
 ] interface Credential {
     readonly attribute DOMString id;
+    readonly attribute DOMString type;
+
+    // TODO(mkwst): These should be on LocallyStoredCredential.
     readonly attribute DOMString name;
     readonly attribute DOMString avatarURL;
 };