Create signin component and componentize TokenWebData.

This CL creates the signin layered component. It additionally componentizes
TokenWebData and moves it into the signin component. The refactorings performed
on TokenWebData are identical to ones performed on AutofillWebDataService:
- threads are injected rather than being obtained from content::BrowserThread
- //chrome code gets the TokenWebData for a Profile via WebDataServiceFactory
  instead of TokenWebData::FromBrowserContext.

BUG=332107
TBR=thakis

Review URL: https://codereview.chromium.org/109783003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243564 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243564