minor code optimization

git-svn-id: svn://svn.code.sf.net/p/htmlunit/code/trunk@8445 5f5364db-9458-4db8-a492-e30667be6df6
WebClient.getTopLevelWindows() returns a snapshot of the list of open top level windows. This avoids ConcurrentModificationExceptions when using this list by a client.

Issue 1534


git-svn-id: svn://svn.code.sf.net/p/htmlunit/code/trunk@8446 5f5364db-9458-4db8-a492-e30667be6df6