Implement -webkit-mask-box-image as a shorthand property

This patch implements -webkit-mask-box-image as a proper shorthand
internally to Blink. This fixes the behaviour of these properties in
inline styles and avoids special case handling to animate the
properties in Web Animations.

As part of making this property expand into its longhand properties
some minor initial value inaccuracies needed to be fixed.
See specs for correct values:
http://www.w3.org/TR/css-masking/#mask-box-image-slice
http://www.w3.org/TR/css-masking/#mask-box-image-width
Usage of these properties is currently ~0.005%, changing behaviour to
match spec will have negligible impact on the web.

Fixes test:
virtual/web-animations-css/animations/cross-fade-webkit-mask-box-image.html

BUG=309962

Review URL: https://codereview.chromium.org/50233002

git-svn-id: svn://svn.chromium.org/blink/trunk@162295 bbb929c8-8fbe-4397-9dbb-9b2b20218538