Deprecate enable-background property

Source/core/css/CSSProperties.in

 // This file specifies all the CSS properties we support and the necessary
 // information for our code generation. The various supported arguments
 // are described below with example usage
 
 
 // - alias_for=other-property
 // Properties specifying alias_for should be virtually identical to the
 // properties they alias. Minor parsing differences are allowed as long as
 // the CSSValues created are of the same format of the aliased property.
 
@@ skipping 411 matching lines @@
 -webkit-logical-height direction_aware
 -webkit-min-logical-width direction_aware
 -webkit-min-logical-height direction_aware
 -webkit-max-logical-width direction_aware
 -webkit-max-logical-height direction_aware
 
 // Properties that we ignore in the StyleBuilder.
 // TODO(timloh): This seems wrong, most of these shouldn't reach the StyleBuilder
 
 all builder_skip
-enable-background builder_skip
 max-zoom builder_skip
 min-zoom builder_skip
 orientation builder_skip
 page builder_skip
 src builder_skip
 unicode-range builder_skip
 user-zoom builder_skip
 -webkit-font-size-delta builder_skip
 -webkit-text-decorations-in-effect inherited, builder_skip
 
@@ skipping 94 matching lines @@
 -webkit-shape-margin alias_for=shape-margin
 -webkit-shape-outside alias_for=shape-outside
 -webkit-transform alias_for=transform
 -webkit-transform-origin alias_for=transform-origin
 -webkit-transform-style alias_for=transform-style
 -webkit-transition alias_for=transition
 -webkit-transition-delay alias_for=transition-delay
 -webkit-transition-duration alias_for=transition-duration
 -webkit-transition-property alias_for=transition-property
 -webkit-transition-timing-function alias_for=transition-timing-function