[css-typed-om] Allow CSSVariableReferenceValue.fallback to be nullable.

Spec change:
w3c/css-houdini-drafts@c9d9c29

Bug: 814487
Change-Id: Idb15d8edc4dc7aba4cf40ac13b69ea13ef7d8f60

css/css-typed-om/stylevalue-subclasses/cssVariableReferenceValue.html

@@ -20,6 +20,16 @@
       'Fallback member should be null');
 }, 'CSSVariableReferenceValue can be constructed with no fallback');
 
+test(() => {
+  const result = new CSSVariableReferenceValue('--foo', null);
+
+  assert_not_equals(result, null,
+      'A CSSVariableReferenceValue should be created');
+  assert_equals(result.variable, '--foo',
+      'Variable member should be same as passed in the constructor');
+  assert_equals(result.fallback, null, 'Fallback member should be null');
+}, 'CSSVariableReferenceValue can be constructed with null fallback');
+
 test(() => {
   const result = new CSSVariableReferenceValue('--foo',
       new CSSUnparsedValue(['lemon']));
@@ -32,7 +42,7 @@
       'Fallback member should not be null');
   assert_style_value_equals(result.fallback, new CSSUnparsedValue(['lemon']),
       'Fallback member should be as same as passed in the constructor');
-}, 'CSSVariableReferenceValue can be constructed with fallback');
+}, 'CSSVariableReferenceValue can be constructed with valid fallback');
 
 test(() => {
   let result = new CSSVariableReferenceValue('--foo');

interfaces/css-typed-om.idl

@@ -54,7 +54,7 @@ interface CSSUnparsedValue : CSSStyleValue {
 typedef (DOMString or CSSVariableReferenceValue) CSSUnparsedSegment;
 
 [Exposed=(Window, Worker, PaintWorklet, LayoutWorklet),
- Constructor(DOMString variable, optional CSSUnparsedValue fallback)]
+ Constructor(DOMString variable, optional CSSUnparsedValue? fallback)]
 interface CSSVariableReferenceValue {
     attribute DOMString variable;
     readonly attribute CSSUnparsedValue? fallback;