Web Animations CSS: Support Animation of StyleImage and LengthBox

This patch adds two new types for animation:
* AnimatableImage implements interpolation of images as defined by web-animations: http://dev.w3.org/fxtf/web-animations/#the--image--type
* AnimatableLengthBox implements interpolation for the LengthBox platform type. This matches the existing implementation with the exception that it allows interpolation between different length types.

Adds support and tests for interpolation of clip. Behavior changes slightly under
Web Animations as we step at 50% when smooth interpolation is not defined (eg. between auto
and 10px). The existing implementation steps at 0% (http://crbug.com/269451). Note that
neither the existing implementation or Web Animations yet support animating a clip when it is 
not set by some other style (http://crbug.com/279684).

A whitelist has been added to allow filtering of properties which do not currently support animation.

BUG=258896,257591

Review URL: https://chromiumcodereview.appspot.com/22799020

git-svn-id: svn://svn.chromium.org/blink/trunk@156846 bbb929c8-8fbe-4397-9dbb-9b2b20218538