'.constructor' in private scripts should not be called multiple times

This is just a subtle bug fix. |isInitialized| should be set when initializeHolderIfNeeded runs at the first time.

BUG=341031

Review URL: https://codereview.chromium.org/380253003

git-svn-id: svn://svn.chromium.org/blink/trunk@177894 bbb929c8-8fbe-4397-9dbb-9b2b20218538