Add HandleScopes and ScriptForbiddenScope::AllowUserAgentScript to private scripts

This is a split CL from https://codereview.chromium.org/394773003/.

- Added HandleScopes because some caller sites don't have HandleScopes.

- Added AllowUserAgentScript because we need to allow script execution to execute private scripts (of course).

BUG=341031

Review URL: https://codereview.chromium.org/419163004

git-svn-id: svn://svn.chromium.org/blink/trunk@178896 bbb929c8-8fbe-4397-9dbb-9b2b20218538