Add new BrowserForceSigninEnabeld policy option with future:true flag.

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index ad942ee506125b8ed5c4eb3efb1e44635304258b..2ecbec2f7e247ea31e4b2400185547ab831999d3 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -2181,6 +2181,9 @@ const char kBrowserGuestModeEnabled[] = "profile.browser_guest_enabled";
 // Whether Adding a new Person is enabled within the user manager.
 const char kBrowserAddPersonEnabled[] = "profile.add_person_enabled";
 
+// Whether profile can be used before sign in.
+const char kForceBrowserSignin[] = "profile.force_browser_signin";
+
 // Device identifier used by Easy Unlock stored in local state. This id will be
 // combined with a user id, before being registered with the CryptAuth server,
 // so it can't correlate users on the same device.