[FastTextAutosizer] Fix crash when list items don't have markers

It's possible for a list item to not have a marker (e.g., using
list-style-type: none;) but FastTextAutosizer::inflateListItem
assumes a list item's marker exists. This patch adds a check that the
list item's marker exists in RenderListItem::layout before calling
the fast text autosizer. Similar checks can be found throughout
RenderListItem.

An existing test has been modified to check for this case.

BUG=342247

Review URL: https://codereview.chromium.org/146603004

git-svn-id: svn://svn.chromium.org/blink/trunk@166788 bbb929c8-8fbe-4397-9dbb-9b2b20218538