Correctly test CSSStyleRule.style.

Remove the assert_readonly test and add one to verify that assigning to
CSSStyleRule.style correctly forwards to CSSStyleRule.style.cssText.

We currently test for whether CSSStyleRule.style is read-only by
trying to assign to it; however, the spec has it as actually:

interface CSSStyleRule : CSSRule {
  ...
  [SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
};

See: https://drafts.csswg.org/cssom/

The `PutForwards=cssText` means that assigning to CSSStyleRule.style
should actually assign to style.cssText.

tests/wpt/metadata/MANIFEST.json

@@ -574763,7 +574763,7 @@
    "testharness"
   ],
   "cssom/CSSStyleRule.html": [
-   "e9d0acfc0c9123dcd2295e217bdfc1ac5195c3f0",
+   "9fe62d2e23709b77e9b5cda4522ec1c04d2940cf",
    "testharness"
   ],
   "cssom/CSSStyleSheet.html": [

tests/wpt/web-platform-tests/cssom/CSSStyleRule.html

@@ -67,7 +67,24 @@
             assert_idl_attribute(rule, "selectorText");
             assert_equals(typeof rule.selectorText, "string");
             assert_idl_attribute(rule, "style");
-        }, "Existence, writability and type of CSSStyleRule attributes");
+        }, "Existence and type of CSSStyleRule attributes");
+
+        test(function() {
+            // CSSStyleRule.style has PutForwards=cssText and SameObject.
+            var initial = rule.style.cssText;
+            var style = rule.style;
+
+            rule.style = "";
+            assert_equals(rule.style.cssText, "");
+            assert_equals(rule.style, style);
+
+            rule.style = "margin: 42px;";
+            assert_equals(rule.style.margin, "42px");
+            assert_equals(rule.style, style);
+
+            rule.style = initial;
+            assert_equals(rule.style, style);
+        }, "Assigning to CSSStyleRule.style assigns to cssText; CSSStyleRule.style returns the same object");
 
         test(function() {
             assert_equals(rule.selectorText, "div");