Rename {first,last}EditablePosition{After,Before}PositionInRoot to denote them returned VisiblePosion

This patch renames {first,last}EditablePosition{After,Before}PositionInRoot to
{first,last}EditableVisiblePosition{After,Before}PositionInRoot for having
lastEditblePositionBeforePositionInRoot() which returns Position instead of
VisiblePosition.

This patch is a preparation of fixing issue 339187 by
http://crrev.com/340713003

BUG=n/a
TEST=n/a

Review URL: https://codereview.chromium.org/340013002

git-svn-id: svn://svn.chromium.org/blink/trunk@176475 bbb929c8-8fbe-4397-9dbb-9b2b20218538