Make ChromeSigninClient a BrowserContextKeyedService

Make ChromeSigninClient a BrowserContextKeyedService

This CL moves ChromeSigninClient from being created in SigninManagerFactory and
owned by SigninManager into being a BCKS. The motivation is that other core
Signin classes (e.g., *ProfileOAuth2TokenService) will need access to the
SigninClient instance as well. ChromeSigninClient being a BCKS enables such
access, as the respective factories can simply get the ChromeSigninClient
instance from the ChromeSigninClientFactory in order to pass it in to the core
classes that are being created.

BUG=350710
R=rogerta@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=255951

[blundell, 2014-03-09 21:11:28]

This CL is dependent on https://codereview.chromium.org/192253002/.

[Roger Tawa OOO till Jul 10th, 2014-03-10 14:47:21]

lgtm

[blundell, 2014-03-10 16:08:40]

Committed patchset #3 manually as r255951 (presubmit successful).