Handle special case of merging lists in mergeParagraphs()

Consider a html content with two consecutive lists. When cursor is placed
at the end of first list and forward delete is executed, we should
merge the second list with the first one.

Currently, the content of the first <li> of second list is appended to
the content of last <li> of first list and two lists are still retained.
This is incorrect. Uploaded patch fixes this erroneous behavior.

Patch introduces the behavior that aligns with that of both FF and IE.

BUG=327624
TEST=LayoutTests/editing/deleting/merge-lists.html

Review URL: https://codereview.chromium.org/112633003

git-svn-id: svn://svn.chromium.org/blink/trunk@165218 bbb929c8-8fbe-4397-9dbb-9b2b20218538