Fixing indent/outdent for <li> elements containing children spanning across
multiple lines (paragraphs).
For such cases the selection only encompasses children contained within the
first paragraph (within the li), thereby causing incorrect behavior when indenting.
Thus we need to ensure while inserting a new li element and cloning the original,
that all the children are considered.

With this change, an existing test: indent-pre-list.html fails because of the
previous incorrect behavior. Two <li>s were created for the indented <li>
with the text contained within the first paragraph forming the first and the
remaining content moving to the second <li> element.
This has now been fixed resulting in only a single indented <li>.

Bug: http://crbug.com/321567

Review URL: https://codereview.chromium.org/67063005

git-svn-id: svn://svn.chromium.org/blink/trunk@162651 bbb929c8-8fbe-4397-9dbb-9b2b20218538