Override SigninManager::SignOut if force-signin is enabled.

components/signin/core/browser/signin_client.h

Index: components/signin/core/browser/signin_client.h
diff --git a/components/signin/core/browser/signin_client.h b/components/signin/core/browser/signin_client.h
index 99b37c49fbb8c8c332922bc556581f9d3c91f75a..2e9a44722a3105cdc6aaa9730c8a1c45dcbf0603 100644
--- a/components/signin/core/browser/signin_client.h
+++ b/components/signin/core/browser/signin_client.h
@@ -34,7 +34,7 @@ class SigninClient : public KeyedService {
   // The subcription for cookie changed notifications.
   class CookieChangedSubscription {
    public:
-    virtual ~CookieChangedSubscription() {};
+    virtual ~CookieChangedSubscription() {}
   };
 
   ~SigninClient() override {}
@@ -97,6 +97,10 @@ class SigninClient : public KeyedService {
                             const std::string& username,
                             const std::string& password) {}
 
+  // Called before Google signout started, call |sign_out| to start the sign out
+  // process.
+  virtual void PreSignOut(const base::Callback<void()>& sign_out);
+
   virtual bool IsFirstRun() const = 0;
   virtual base::Time GetInstallDate() = 0;
 
@@ -120,6 +124,9 @@ class SigninClient : public KeyedService {
       const std::string& source,
       net::URLRequestContextGetter* getter) = 0;
 
+  // Called once the credentials has been copied to another SigninManager.
+  virtual void OnCredentialsBeingCopied() {}

[Roger Tawa OOO till Jul 10th, 2016/11/02 13:20:39]

Could this be renamed to OnCredentialsCopied() ?  With "Being" in the name it's
not clear if this is called before, during, or after the copy.  Could also
replace "once" with "after" too.

[zmin, 2016/11/02 20:30:22]

Done.

+
  protected:
   // Returns device id that is scoped to single signin.
   // Stores the ID in the kGoogleServicesSigninScopedDeviceId pref.