Componentize SigninManager.
This CL does the following:
- Componentizes SigninIdAccountHelper, on which SigninManager depends
- Abstracts SigninManager's (embedder-level) calculation of whether to merge
signin credentials into the cookie jar via SigninClient
- Eliminates the |profile| argument of SigninManagerBase::Initialize() now that
SigninManager no longer needs it
- Componentizes SigninManager
BUG=334209,333998
TBR=thakis
Review URL: https://codereview.chromium.org/
218713002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260636 0039d316-1c4b-4281-b951-d872f2087c98