[login_manager] Use keygen helper to generate owner key

owner_key.h

Index: owner_key.h
diff --git a/owner_key.h b/owner_key.h
index ed7bf144334ed8b93fb6350da9731a827df77ba5..9335b8bcf4c2488c3cde34a914fe6353e5c7fac5 100644
--- a/owner_key.h
+++ b/owner_key.h
@@ -17,6 +17,7 @@ class SignatureVerifier;
 }  // namespace base
 
 namespace login_manager {
+class ChildJobInterface;
 class SystemUtils;
 
 // This class holds the device owner's public key.
@@ -72,6 +73,9 @@ class OwnerKey {
                     uint32 data_len,
                     std::vector<uint8>* OUT_signature);
 
+  // Runs |generator| in a child process.  Returns pid of the child.
+  virtual int StartGeneration(ChildJobInterface* generator);
+
   // Returned reference will be empty if we haven't populated |key_| yet.
   const std::vector<uint8>& public_key_der() const {
     return key_;