Remove all unsafe usage of FakeSigninManagerBase.

Several cross-platform tests were supplying FakeSigninManagerBase::Build() as
the build function for SigninManagerFactory. This is unsafe, as on all
platforms other than ChromeOS SigninManagerFactory::GetForProfile() returns a
SigninManager* (which has an expanded public API over SigninManagerBase). This
CL changes FakeSigninManagerBase::Build() to create a FakeSigninManagerBase on
ChromeOS and a FakeSigninManager on all other platforms (similar to the way
that SigninManagerFactory::GetForProfile() works). It changes all clients to
use FakeSigninManagerBase::Build().

TBR=zea

Review URL: https://codereview.chromium.org/136533003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246631 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246631