Make DOMSelection::setBaseAndExtent() not to use createLegacyEditingPosition()
Make DOMSelection::setBaseAndExtent() not to use createLegacyEditingPosition()

This patch make |DOMSelection::setBaseAndExtent()|to use |Position| constructor
instead of |createLegacyEditingPosition()| to avoid using legacy function for
improving code health, since |VisiblePostion| is same for both legacy editing
position and non-legacy editing position due by canonicalization in
|VisiblePostion| constructor.

This patch is a preparation of getting rid of legacy editing position.

BUG=513114
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/1262203004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@199769 bbb929c8-8fbe-4397-9dbb-9b2b20218538

Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk