Settings: simplify people_handler start-signin logic

When HandleStartSignin is called, we know for sure authentication is needed,
so its unnecessary to call OpenSyncSetup(), which will go through all the setup
work for sync, realizing authentication is needed, then undo all the setup work
before bringing up GAIA UI.

Bug: 800972
Change-Id: I0b38d90dd4b5a1f9c0fc8dfc6afa607c2b27e616
Reviewed-on: https://chromium-review.googlesource.com/967244
Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544174}

chrome/browser/ui/webui/settings/people_handler.cc

@@ -653,7 +653,7 @@ void PeopleHandler::HandleStartSignin(const base::ListValue* args) {
   // error.
   DCHECK(IsProfileAuthNeeded());
 
-  OpenSyncSetup();
+  DisplayGaiaLogin(signin_metrics::AccessPoint::ACCESS_POINT_SETTINGS);
 }
 
 void PeopleHandler::HandleStopSyncing(const base::ListValue* args) {