Add support for the object-position CSS property.

This is hidden behind an "experimental" runtime flag named
"ObjectFitPosition", together with object-fit.

This is an implementation of object-position as described in
http://www.w3.org/TR/2012/CR-css3-images-20120417/#object-position

Object-position is used to offset replaced content within its content
box. Painting is always clipped against the content box, regardless of
the "overflow" property. This property is useful together with
object-fit (to achieve a difference between content box size and
replaced content size, so that specifying alignment is interesting),
but can also be used on its own.

BUG=236333

Review URL: https://chromiumcodereview.appspot.com/22839023

git-svn-id: svn://svn.chromium.org/blink/trunk@157745 bbb929c8-8fbe-4397-9dbb-9b2b20218538