Share implementation of queryCommand for InsertOrderedList and InsertUnorderedList

This patch unifies Editor::selection{Ordered,Unordered}ListState() functions
into static function selectionListState() in "core/editing/EditorCommand.cpp"
to reduce code size and improve maintainability. Because original
implementations are same except for tag name to check, "OL" or "UL".

BUG=n/a
TEST=n/a; no behavior change

Review URL: https://codereview.chromium.org/332083004

git-svn-id: svn://svn.chromium.org/blink/trunk@176722 bbb929c8-8fbe-4397-9dbb-9b2b20218538