Improve drag&drop of list items in contentEditable divs
https://bugs.webkit.org/show_bug.cgi?id=111556

Reviewed by Ryosuke Niwa.

Source/WebCore:

A single fully selected <li> should generate the same markup as if
we were selecting some of them, i.e., we should preserve the list
structure and appearance.

Test: editing/selection/drag-list-item.html

* editing/htmlediting.cpp:
(WebCore::isListItem): modified constness.
* editing/htmlediting.h:
(WebCore): ditto.
* editing/markup.cpp:
(WebCore::highestAncestorToWrapMarkup):

LayoutTests:

Added a new test case. The test performs 2 drag&drops, first of
all a selection of a single list item, and secondly a selection of
two list items. It checks that both behave the same way, i.e, the
whole <li> is moved and not only the text.

* editing/selection/drag-list-item-expected.txt: Added.
* editing/selection/drag-list-item.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145195 268f45cc-cd09-0410-ab3c-d52691b4dbfc