Revert 157745 "Add support for the object-position CSS property."

This is causing a number of debug layout tests to crash with
"ASSERTION FAILED: object-position is not whitelisted for animation"
in CSSAnimations::isAnimatableProperty.
e.g. see
http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20%28dbg%29/builds/12296/steps/webkit_tests/logs/stdio

> 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

TBR=mstensho@opera.com

Review URL: https://codereview.chromium.org/23465021

git-svn-id: svn://svn.chromium.org/blink/trunk@157751 bbb929c8-8fbe-4397-9dbb-9b2b20218538