Make CSSRule.PROPERTY_RULE = 18 as per spec.

https://drafts.css-houdini.org/css-properties-values-api/#extensions-to-css-rule-interface

Bug: 1008886
Change-Id: Ie587ec75864b856b2c020ba28ce209718c772c86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1875256Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708575}

third_party/blink/renderer/core/css/css_rule.h

@@ -56,11 +56,9 @@ class CORE_EXPORT CSSRule : public ScriptWrappable {
     kNamespaceRule = 10,
     kSupportsRule = 12,
     kViewportRule = 15,
+    kPropertyRule = 18,
     // Experimental features below. Such features must be greater than 1000:
     // the 0-1000 range is reserved by the CSS Working Group.
-    //
-    // TODO(https://crbug.com/978781): Spec a proper number.
-    kPropertyRule = 1001,
   };
 
   virtual Type type() const = 0;

third_party/blink/renderer/core/css/css_rule.idl

@@ -51,6 +51,6 @@
     [RuntimeEnabled=CSSViewport] const unsigned short VIEWPORT_RULE = 15;
 
     // CSS Properties and Values Level 1
-    // TODO(https://crbug.com/978781): Spec a proper number.
-    [RuntimeEnabled=CSSVariables2AtProperty] const unsigned short PROPERTY_RULE = 1001;
+    // https://drafts.css-houdini.org/css-properties-values-api/#extensions-to-css-rule-interface
+    [RuntimeEnabled=CSSVariables2AtProperty] const unsigned short PROPERTY_RULE = 18;
 };

third_party/blink/web_tests/virtual/at-property/external/wpt/css/css-properties-values-api/idlharness-expected.txt

@@ -15,8 +15,8 @@ FAIL CSSPropertyRule interface: attribute name assert_true: The prototype object
 FAIL CSSPropertyRule interface: attribute syntax assert_true: The prototype object must have a property "syntax" expected true got false
 FAIL CSSPropertyRule interface: attribute inherits assert_true: The prototype object must have a property "inherits" expected true got false
 FAIL CSSPropertyRule interface: attribute initialValue assert_true: The prototype object must have a property "initialValue" expected true got false
-FAIL CSSRule interface: constant PROPERTY_RULE on interface object assert_equals: property has wrong value expected 18 but got 1001
-FAIL CSSRule interface: constant PROPERTY_RULE on interface prototype object assert_equals: property has wrong value expected 18 but got 1001
+PASS CSSRule interface: constant PROPERTY_RULE on interface object
+PASS CSSRule interface: constant PROPERTY_RULE on interface prototype object
 PASS CSS namespace: operation escape(CSSOMString)
 PASS CSS namespace: operation registerProperty(PropertyDefinition)
 Harness: the test ran to completion.