Avoid anchoring to pseudo elements when using logical ordering in a selection

When we start the selection range in an element the selection is taken to include
its :before or :after pseudoelement. So likewise when we modify the selection to an 
element on a line boundary that selection should include its :before/:after 
pseduoelements.

BUG=342574

Review URL: https://codereview.chromium.org/693283002

git-svn-id: svn://svn.chromium.org/blink/trunk@184798 bbb929c8-8fbe-4397-9dbb-9b2b20218538