Fix null pointer dereference in CompositeEditCommand::moveParagraphs().

Document::documentElement() may return null, and in that case we should not
hand it to PlainTextRange::createRangeForSelection().

BUG=338542

Review URL: https://codereview.chromium.org/176863002

git-svn-id: svn://svn.chromium.org/blink/trunk@167684 bbb929c8-8fbe-4397-9dbb-9b2b20218538