Clear GCM data when the user clears cookies and other site data

components/gcm_driver/gcm_driver.h

Index: components/gcm_driver/gcm_driver.h
diff --git a/components/gcm_driver/gcm_driver.h b/components/gcm_driver/gcm_driver.h
index c10d9f16769ddfc445e1ce1c9a2a16b7fad7469a..98780688cee148bb87c449803a44100456bea3e1 100644
--- a/components/gcm_driver/gcm_driver.h
+++ b/components/gcm_driver/gcm_driver.h
@@ -72,9 +72,9 @@ class GCMDriver {
   // been called, no other GCMDriver methods may be used.
   virtual void Shutdown();
 
-  // Call this method when the user signs in to a GAIA account.
-  // TODO(jianli): To be removed when sign-in enforcement is dropped.
+  // Called when the user signs in to or out of a GAIA account.
   virtual void OnSignedIn() = 0;
+  virtual void OnSignedOut() = 0;
 
   // Removes all the cached and persisted GCM data. If the GCM service is
   // restarted after the purge, a new Android ID will be obtained.