Implements CSSPropertyAPI for the webkit-box-flex-group 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.
 
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 -webkit-app-region custom_all
 -webkit-background-clip use_handlers_for=CSSPropertyBackgroundClip
 -webkit-background-origin use_handlers_for=CSSPropertyBackgroundOrigin
 -webkit-border-horizontal-spacing interpolable, inherited, name_for_methods=HorizontalBorderSpacing, converter=convertComputedLength<short>
 -webkit-border-image initial=initialNinePieceImage, custom_value
 -webkit-border-vertical-spacing interpolable, inherited, name_for_methods=VerticalBorderSpacing, converter=convertComputedLength<short>
 -webkit-box-align type_name=EBoxAlignment
 -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-flex-group type_name=unsigned int, api_class
 -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, api_class
 column-rule-color interpolable, custom_all
 column-rule-style type_name=EBorderStyle, initial=initialBorderStyle
 column-rule-width interpolable, converter=convertLineWidth<unsigned short>
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 -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