Add a generated ComputedStyleBase class that ComputedStyle extends

third_party/WebKit/Source/core/css/CSSProperties.in

Index: third_party/WebKit/Source/core/css/CSSProperties.in
diff --git a/third_party/WebKit/Source/core/css/CSSProperties.in b/third_party/WebKit/Source/core/css/CSSProperties.in
index 6dc3b0d7b0b6b43561def83d8d0d8071dd585bd5..64833a86ecd32d2b63b8ec2a7863978cd837daf4 100644
--- a/third_party/WebKit/Source/core/css/CSSProperties.in
+++ b/third_party/WebKit/Source/core/css/CSSProperties.in
@@ -22,13 +22,23 @@
 // - longhands=property;other-property
 // The property is a shorthand for several other properties.
 
+// keyword_only
+// These properties only store keyword values. This is used when
+// generating the ComputedStyle storage for the property. The initial
+// value for this property on a ComputedStyle is the first keyword in
+// the specified list of keywords.
+// TODO(sashab, meade): Remove this once TypedOM types are specified for
+// every property, since this value can be inferred from that.
+
+// - keywords=[keyword1|keyword2]
+// This specifies all valid keyword values for the property.
+// TODO(sashab): Once all properties are represented here, delete
+// CSSValueKeywords.in and use this list instead.
 
 // Flags which go into CSSOMTypes:
 // - typedom_types=[Type|OtherType]
 // The property can take types specified in typedom_types for CSS Typed OM.
 // Keyword does not need to be specified as every property can take keywords.
-// - keywords=[keyword1|keyword2]
-// The property can take these keywords.
 // - supports_multiple
 // The property supports a list of values.
 // - supports_percentage
@@ -336,7 +346,7 @@ transition-timing-function custom_all
 unicode-bidi
 vector-effect svg
 vertical-align interpolable, custom_inherit, custom_value
-visibility interpolable, inherited
+visibility interpolable, inherited, keyword_only, keywords=[visible|hidden|collapse]
 x interpolable, svg, converter=convertLength
 y interpolable, svg, converter=convertLength
 -webkit-appearance type_name=ControlPart