Merge 182705

Merge 182705
> Properly handle disabling sync via the google dashboard.
> 
> Add SigninManager::ProhibitSignout() that the policy code can use to disable
> signout. This makes our previous ad-hoc signout prohibition more stable by
> catching other places where we might sign out (such as cancelling sync during
> re-auth).
> 
> Add SigninManager::IsSignoutProhibited() which UI code that wants to show/hide
> signout UI can call.
> 
> Restores old behavior where stopping sync via the google dashboard will try to
> sign out the user.
> 
> Fixed sync UI to properly display the "Sync is stopped via the google dashboard"
> message when sync is stopped on desktop platforms.
> 
> Updated sync settings code to trigger a re-auth flow when the user clicks on "Advanced Sync Setup..." when sync is stopped via the dashboard.
> 
> BUG=174134
> 
> Review URL: https://codereview.chromium.org/12258036

TBR=atwilson@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=183434

[Andrew T Wilson (Slow), 2013-02-20 08:34:04]

Committed patchset #1 manually as r183434.