Allow animation of non-interpolable properties in CSS Animations

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.
 
 
 // - runtime_flag=CSSGridLayout
 // The flag on RuntimeEnabledFeatures conditionally enables the property.
 // This doesn't currently work with alias_for.
 
 
 // - longhands=property;other-property
 // The property is a shorthand for several other properties.
 
 
 // Flags which go into CSSPropertyMetadata:
 // - interpolable
-// The interpolable flag indicates whether a property can be animated by CSS
-// animations and transitions. If this flag is set, the property should also be
-// added to the switch statements in AnimatedStyleBuilder, CSSPropertyEquality,
-// and CSSAnimatableValueFactory.
+// The interpolable flag indicates whether a property can be animated smoothly.
+// If this flag is set, the property should also be added to the switch
+// statements in AnimatedStyleBuilder, CSSPropertyEquality and
+// CSSAnimatableValueFactory.
 // - inherited
 // The property will inherit by default if no value is specified, typically
 // mentioned in specifications as "Inherited: yes"
 
 
 // The remaining arguments are used for the StyleBuilder and allow us to
 // succinctly describe how to apply properties. When default handlers are not
 // sufficient, we should prefer to use converter, and failing that define
 // custom property handlers in StyleBuilderCustom.cpp. We only should use
 // StyleBuilderFunctions.cpp.tmpl to define handlers when there are multiple
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 -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