Implements CSSPropertyAPI for the column-gap property.

third_party/WebKit/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 399 matching lines @@
 -webkit-box-decoration-break
 -webkit-box-direction inherited, independent, keyword_only, keywords=[normal|reverse], initial_keyword=normal
 -webkit-box-flex type_name=float
 -webkit-box-flex-group type_name=unsigned int
 -webkit-box-lines
 -webkit-box-ordinal-group type_name=unsigned int
 -webkit-box-orient
 -webkit-box-pack
 -webkit-box-reflect converter=convertBoxReflect
 column-count interpolable, type_name=unsigned short, custom_all
-column-gap interpolable, converter=convertComputedLength<float>, custom_all
+column-gap interpolable, converter=convertComputedLength<float>, custom_all, api_class
 column-rule-color interpolable, custom_all
 column-rule-style type_name=EBorderStyle, initial=initialBorderStyle
 column-rule-width interpolable, converter=convertLineWidth<unsigned short>
 column-span type_name=ColumnSpan
 column-width interpolable, converter=convertComputedLength<float>, custom_all
 -webkit-highlight inherited, converter=convertString<CSSValueNone>
 -webkit-hyphenate-character inherited, name_for_methods=HyphenationString, converter=convertString<CSSValueAuto>
 -webkit-line-break inherited, type_name=LineBreak
 -webkit-line-clamp type_name=LineClampValue
 -webkit-margin-after-collapse type_name=EMarginCollapse
@@ skipping 219 matching lines @@
 -webkit-text-size-adjust alias_for=text-size-adjust
 -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
 -webkit-user-select alias_for=user-select