[Sync] Migrate SyncInternalsMessageHandler off CallJavascriptFunctionUnsafe.

components/sync/driver/sync_service.h

Index: components/sync/driver/sync_service.h
diff --git a/components/sync/driver/sync_service.h b/components/sync/driver/sync_service.h
index 84691ad796d03919bdcce604d46d6dce0414d9cc..1b323b2f5a077b3eff88908d8421c0372764d463 100644
--- a/components/sync/driver/sync_service.h
+++ b/components/sync/driver/sync_service.h
@@ -20,6 +20,7 @@
 #include "google_apis/gaia/google_service_auth_error.h"
 
 class GoogleServiceAuthError;
+class SigninManagerBase;
 
 namespace sync_sessions {
 class OpenTabsUIDelegate;
@@ -347,6 +348,10 @@ class SyncService : public DataTypeEncryptionHandler {
       const base::Callback<void(std::unique_ptr<base::ListValue>)>&
           callback) = 0;
 
+  // Non-owning pointer to sign in logic that can be used to fetch information
+  // about the currently signed in user.
+  virtual SigninManagerBase* signin() const = 0;
+
  protected:
   SyncService() {}