Fix performance regression of tiny-innerHTML.html caused by r176901

The reason of the performance regressions was I added the following check to ScriptController::canExecuteScripts()

if (!context.IsEmpty() && toDOMWindow(context) && DOMWrapperWorld::current(m_isolate).isPrivateScriptIsolatedWorld()) {
    return true;
}

This CL moves this check to the last of canExecuteScripts(). In other words, the check runs only when canExecuteScripts() is going to return false. This will fix the performance regression since tiny-innerHTML.html always return true for canExecuteScripts().

BUG=389287

Review URL: https://codereview.chromium.org/367613004

git-svn-id: svn://svn.chromium.org/blink/trunk@177305 bbb929c8-8fbe-4397-9dbb-9b2b20218538