[cssom] [css-page] Add orientation to CSSPageDescriptors

Author: AlaskanEmily

cssom-1/Overview.bs

@@ -2120,6 +2120,7 @@ interface CSSPageDescriptors : CSSStyleDeclaration {
   attribute [LegacyNullToEmptyString] CSSOMString margin-bottom;
   attribute [LegacyNullToEmptyString] CSSOMString margin-left;
   attribute [LegacyNullToEmptyString] CSSOMString size;
+  attribute [LegacyNullToEmptyString] CSSOMString orientation;
   attribute [LegacyNullToEmptyString] CSSOMString marks;
   attribute [LegacyNullToEmptyString] CSSOMString bleed;
 };