Add a mojo interface for AccountId

components/signin/core/account_id/account_id.h

Index: components/signin/core/account_id/account_id.h
diff --git a/components/signin/core/account_id/account_id.h b/components/signin/core/account_id/account_id.h
index 4bc02054a86caede0d828110db4523693b789483..cc171b5c79e4265816c887cd41c524759a41a354 100644
--- a/components/signin/core/account_id/account_id.h
+++ b/components/signin/core/account_id/account_id.h
@@ -24,6 +24,7 @@ class AccountId {
  public:
   struct EmptyAccountId;
 
+  AccountId();
   AccountId(const AccountId& other);
 
   // If any of the comparable AccountIds has AccountType == UNKNOWN then it
@@ -90,7 +91,6 @@ class AccountId {
                           AccountId* out_account_id);
 
  private:
-  AccountId();
   AccountId(const std::string& id,
             const std::string& user_email,
             const AccountType& account_type);