Expose passwords to JavaScript in Credential Manager API

This change implements the proposed change to the Credential Manager API
to directly expose passwords to JavaScript. It also deprecates the
existing PasswordCredential attributes and the ability to attach a
Credential to fetch.

See the corresponding GitHub issue and pull request for more details:
- w3c/webappsec-credential-management#75
- w3c/webappsec-credential-management#76

- Intent to Deprecate: Custom fetch for Credential Manager API:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/WPckvg8JO0U

- Intent to Implement and Ship: The password property of PasswordCredential:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/nT51eE7fWn4

BUG=718416

Review-Url: https://codereview.chromium.org/2852423002
Cr-Commit-Position: refs/heads/master@{#473576}

credential-management/credentialscontainer-create-basics.https.html

@@ -25,7 +25,7 @@
             assert_equals(credential.name, '');
             assert_equals(credential.iconURL, '');
             assert_equals(credential.type, 'password');
-            // TODO(jdoerrie): Add checks for 'credential.password'.
+            assert_equals(credential.password, 'pencil');
         });
 }, "navigator.credentials.create() with valid PasswordCredentialData");
 
@@ -43,6 +43,7 @@
             assert_equals(credential.name, 'friendly name');
             assert_equals(credential.iconURL, 'https://example.com/photo');
             assert_equals(credential.type, 'password');
+            assert_equals(credential.password, 'sekrit');
         });
 }, "navigator.credentials.create() with valid HTMLFormElement");
 

credential-management/idl.https.html

@@ -75,10 +75,7 @@
      Exposed=Window,
      SecureContext]
     interface PasswordCredential : Credential {
-      attribute USVString idName;
-      attribute USVString passwordName;
-
-      attribute CredentialBodyType? additionalData;
+      readonly attribute DOMString password;
     };
     PasswordCredential implements CredentialUserData;