Reviewed by darin
        
        <rdar://problem/5136770> 
        Gmail Editor: Hang when turning a particular multi-line selection into a list

        Demonstrates the bug:
        * editing/execCommand/5136770-expected.checksum: Added.
        * editing/execCommand/5136770-expected.png: Added.
        * editing/execCommand/5136770-expected.txt: Added.
        * editing/execCommand/5136770.html: Added.
        
        Added placeholders in empty list items:
        * editing/execCommand/4747450-expected.txt:
        * editing/execCommand/insert-list-empty-div-expected.txt:
        * editing/execCommand/4917055-expected.txt:
        
        A horizontal rule pushed into a list item appears *before*
        the list marker in the render tree, although, the new results 
        look more correct (13376):
        * editing/execCommand/create-list-with-hr-expected.checksum:
        * editing/execCommand/create-list-with-hr-expected.png:
        * editing/execCommand/create-list-with-hr-expected.txt:
        * editing/execCommand/create-list-with-hr.html:

WebCore:

        Reviewed by darin

        <rdar://problem/5136770> 
        Gmail Editor: Hang when turning a particular multi-line selection into a list
        
        When InsertListCommand pushes content into list items, 
        it creates an empty list item and then calls moveParagraphs.  
        But moveParagraphs' selection preservation code fails when 
        it encounters empty list items (list items w/o placeholders).
        This causes InsertListCommand to lose track of where it has 
        already been performed, which causes the hang.

        * editing/InsertListCommand.cpp:
        (WebCore::InsertListCommand::doApply): Use brs to hold open 
        empty list items.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@20923 268f45cc-cd09-0410-ab3c-d52691b4dbfc