Provide iOS implementation of BookmarkServerClusterService factory

Provide iOS implementation of BookmarkServerClusterService factory

Expand the KeyedServiceProvider interface to allow creation of KeyedServices
required by BookmarkServerClusterService constructor and to give access to
the corresponding factories (for dependencies).

Implement iOS version of BookmarkServerClusterService factory and change the
iOS code to use that implementation instead of the one from //chrome.

BUG=429756
Committed: https://crrev.com/61088dc6bf37c7480199561d11f4cf14cf696bb6
Cr-Commit-Position: refs/heads/master@{#332375}

[sdefresne, 2015-06-01 18:53:53]

sdefresne@chromium.org changed reviewers:
+ droger@chromium.org

[sdefresne, 2015-06-01 18:53:54]

Please take a look.

[droger, 2015-06-02 08:06:03]

lgtm

[sdefresne, 2015-06-02 12:21:29]

The CQ bit was checked by sdefresne@chromium.org

[commit-bot: I haz the power, 2015-06-02 12:21:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1159743005/1

[commit-bot: I haz the power, 2015-06-02 12:48:59]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-02 12:49:38]

Patchset 1 (id:??) landed as
https://crrev.com/61088dc6bf37c7480199561d11f4cf14cf696bb6
Cr-Commit-Position: refs/heads/master@{#332375}