CREDENTIAL: Serialize 'PasswordCredential' objects with "" as the federation.

CREDENTIAL: Serialize 'PasswordCredential' objects with "" as the federation.

After https://codereview.chromium.org/1723583004, we serialize
`PasswordCredential` objects with a `federation_url` of "null" (as "null" is
the serialization of an empty/unique origin.

Neither Android nor Vasilii like this. This patch special-cases things to
ensure that Android (and Vasilii) remains as happy as can be.

BUG=591310
R=vasilii@chromium.org

Review URL: https://codereview.chromium.org/1755053002

Cr-Commit-Position: refs/heads/master@{#378734}
(cherry picked from commit 45bf9a3d72344f1b7a6f28e0bc1c5536339db4a8)

Committed: https://chromium.googlesource.com/chromium/src/+/0e8fd0d85b3174359835662c3dbd9f3059120729

[Mike West, 2016-03-03 09:09:09]

Description was changed from

==========
CREDENTIAL: Serialize 'PasswordCredential' objects with "" as the federation.

After https://codereview.chromium.org/1723583004, we serialize
`PasswordCredential` objects with a `federation_url` of "null" (as "null" is
the serialization of an empty/unique origin.

Neither Android nor Vasilii like this. This patch special-cases things to
ensure that Android (and Vasilii) remains as happy as can be.

BUG=591310
R=vasilii@chromium.org

Review URL: https://codereview.chromium.org/1755053002

Cr-Commit-Position: refs/heads/master@{#378734}
(cherry picked from commit 45bf9a3d72344f1b7a6f28e0bc1c5536339db4a8)
==========

to

==========
CREDENTIAL: Serialize 'PasswordCredential' objects with "" as the federation.

After https://codereview.chromium.org/1723583004, we serialize
`PasswordCredential` objects with a `federation_url` of "null" (as "null" is
the serialization of an empty/unique origin.

Neither Android nor Vasilii like this. This patch special-cases things to
ensure that Android (and Vasilii) remains as happy as can be.

BUG=591310
R=vasilii@chromium.org

Review URL: https://codereview.chromium.org/1755053002

Cr-Commit-Position: refs/heads/master@{#378734}
(cherry picked from commit 45bf9a3d72344f1b7a6f28e0bc1c5536339db4a8)

Committed:
https://chromium.googlesource.com/chromium/src/+/0e8fd0d85b3174359835662c3dbd...
==========

[Mike West, 2016-03-03 09:09:11]

Committed patchset #1 (id:1) manually as
0e8fd0d85b3174359835662c3dbd9f3059120729.