Add UseCounter for unknown namespace usage in selectors.

R=chrishtr@chromium.org

Bug: 901938
Change-Id: I9e20fe19f2c97b98f8250c6c9914512b5a73d8c7
Reviewed-on: https://chromium-review.googlesource.com/c/1318891
Commit-Queue: Anders Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606845}

third_party/blink/public/platform/web_feature.mojom

@@ -2075,6 +2075,7 @@ enum WebFeature {
   kUpdateWithoutShippingOptionOnShippingOptionChange = 2623,
   kCSSSelectorEmptyWhitespaceOnlyFail = 2624,
   kActivatedImplicitRootScroller = 2625,
+  kCSSUnknownNamespacePrefixInSelector = 2626,
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.
   // Also, run update_use_counter_feature_enum.py in

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

@@ -324,6 +324,7 @@ std::unique_ptr<CSSParserSelector> CSSSelectorParser::ConsumeCompoundSelector(
   if (!compound_selector) {
     AtomicString namespace_uri = DetermineNamespace(namespace_prefix);
     if (namespace_uri.IsNull()) {
+      context_->Count(WebFeature::kCSSUnknownNamespacePrefixInSelector);
       failed_parsing_ = true;
       return nullptr;
     }

third_party/blink/renderer/core/frame/use_counter_test.cc

@@ -415,4 +415,27 @@ TEST_F(DeprecationTest, InspectorDisablesDeprecation) {
   EXPECT_TRUE(use_counter_.HasRecordedMeasurement(feature));
 }
 
+TEST_F(UseCounterTest, CSSUnknownNamespacePrefixInSelector) {
+  std::unique_ptr<DummyPageHolder> dummy_page_holder =
+      DummyPageHolder::Create(IntSize(800, 600));
+  Page::InsertOrdinaryPageForTesting(&dummy_page_holder->GetPage());
+  Document& document = dummy_page_holder->GetDocument();
+  WebFeature feature = WebFeature::kCSSUnknownNamespacePrefixInSelector;
+  EXPECT_FALSE(UseCounter::IsCounted(document, feature));
+
+  document.documentElement()->SetInnerHTMLFromString(R"HTML(
+    <style>
+      @namespace svg url(http://www.w3.org/2000/svg);
+      svg|a {}
+      a {}
+    </style>
+  )HTML");
+  document.View()->UpdateAllLifecyclePhases();
+  EXPECT_FALSE(UseCounter::IsCounted(document, feature));
+
+  document.documentElement()->SetInnerHTMLFromString("<style>foo|a {}</style>");
+  document.View()->UpdateAllLifecyclePhases();
+  EXPECT_TRUE(UseCounter::IsCounted(document, feature));
+}
+
 }  // namespace blink

tools/metrics/histograms/enums.xml

@@ -20696,6 +20696,7 @@ Called by update_net_error_codes.py.-->
   <int value="2623" label="UpdateWithoutShippingOptionOnShippingOptionChange"/>
   <int value="2624" label="CSSSelectorEmptyWhitespaceOnlyFail"/>
   <int value="2625" label="ActivatedImplicitRootScroller"/>
+  <int value="2626" label="CSSUnknownNamespacePrefixInSelector"/>
 </enum>
 
 <enum name="FeaturePolicyFeature">