Move PositionWithAffinity class to editing

This patch moves PositionWithAffinity class to editing/ from dom/Position.h
since it is "affinity" is editing concept rather than DOM concept.

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

Review URL: https://codereview.chromium.org/353603003

git-svn-id: svn://svn.chromium.org/blink/trunk@177035 bbb929c8-8fbe-4397-9dbb-9b2b20218538