[@property] Actually count usage + unit test

Bug: 1085994
Change-Id: Ied0e00fa9e67850860ed0126269533396834b66b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2215814Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771987}

third_party/blink/renderer/core/css/parser/css_at_rule_id.cc

@@ -63,7 +63,7 @@ void CountAtRule(const CSSParserContext* context, CSSAtRuleID rule_id) {
       break;
     case kCSSAtRuleProperty:
       feature = WebFeature::kCSSAtRuleProperty;
-      return;
+      break;
     case kCSSAtRuleSupports:
       feature = WebFeature::kCSSAtRuleSupports;
       break;

third_party/blink/renderer/core/css/style_engine_test.cc

@@ -2702,6 +2702,30 @@ TEST_F(StyleEngineTest, PrintNoDarkColorScheme) {
                                     GetCSSPropertyColor()));
 }
 
+TEST_F(StyleEngineTest, AtPropertyUseCount) {
+  ScopedCSSVariables2AtPropertyForTest scoped_feature(true);
+
+  GetDocument().body()->setInnerHTML(R"HTML(
+    <style>
+      body { --x: No @property rule here; }
+    </style>
+  )HTML");
+  UpdateAllLifecyclePhases();
+  EXPECT_FALSE(GetDocument().IsUseCounted(WebFeature::kCSSAtRuleProperty));
+
+  GetDocument().body()->setInnerHTML(R"HTML(
+    <style>
+      @property --x {
+        syntax: "<length>";
+        inherits: false;
+        initial-value: 0px;
+      }
+    </style>
+  )HTML");
+  UpdateAllLifecyclePhases();
+  EXPECT_TRUE(GetDocument().IsUseCounted(WebFeature::kCSSAtRuleProperty));
+}
+
 class ParameterizedStyleEngineTest
     : public testing::WithParamInterface<bool>,
       private ScopedCSSReducedFontLoadingInvalidationsForTest,