signin: pull basic SigninManager functionality into new SigninManagerBase class.

chrome/browser/signin/signin_tracker.cc

Index: chrome/browser/signin/signin_tracker.cc
diff --git a/chrome/browser/signin/signin_tracker.cc b/chrome/browser/signin/signin_tracker.cc
index 689717b8a6222eb7a8c0b47d6858fc1be4681ba6..46fbcbb0742c048b2b1847e145093060eaeb4756 100644
--- a/chrome/browser/signin/signin_tracker.cc
+++ b/chrome/browser/signin/signin_tracker.cc
@@ -133,7 +133,7 @@ void SigninTracker::HandleServiceStateChange() {
     return;
   }
 
-  SigninManager* signin = SigninManagerFactory::GetForProfile(profile_);
+  SigninManagerBase* signin = SigninManagerFactory::GetForProfile(profile_);
   if (signin->GetAuthenticatedUsername().empty()) {
     // User is signed out, trigger a signin failure.
     state_ = WAITING_FOR_GAIA_VALIDATION;