Fix selector namespace prefix resolution.

third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp

Index: third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp
diff --git a/third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp b/third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp
index be1b17a26a02cabb1a8b16428e29047761661e2a..4f72a0adc2f72aa92fb004c5f61ea56310c7162a 100644
--- a/third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp
+++ b/third_party/WebKit/Source/core/css/parser/CSSSelectorParser.cpp
@@ -304,8 +304,10 @@ PassOwnPtr<CSSParserSelector> CSSSelectorParser::consumeCompoundSelector(CSSPars
 
     if (!compoundSelector) {
         AtomicString namespaceURI = determineNamespace(namespacePrefix);
-        if (namespaceURI.isNull())
+        if (namespaceURI.isNull()) {
+            m_failedParsing = true;

[rune, 2016/01/22 11:50:30]

There's a separate CL (https://codereview.chromium.org/1625433002/) for this,
but I needed the fix to not have regressions in the exception throwing for
querySelector and friends.

             return nullptr;
+        }
         return CSSParserSelector::create(QualifiedName(namespacePrefix, elementName, namespaceURI));
     }
     prependTypeSelectorIfNeeded(namespacePrefix, elementName, compoundSelector.get());
@@ -701,9 +703,15 @@ const AtomicString& CSSSelectorParser::defaultNamespace() const
 
 const AtomicString& CSSSelectorParser::determineNamespace(const AtomicString& prefix)
 {
-    if (!m_styleSheet)
+    if (prefix.isNull())
         return defaultNamespace();
-    return m_styleSheet->determineNamespace(prefix);
+    if (prefix.isEmpty())
+        return emptyAtom; // No namespace. If an element/attribute has a namespace, we won't match it.
+    if (prefix == starAtom)
+        return starAtom; // We'll match any namespace.
+    if (!m_styleSheet)
+        return nullAtom; // Cannot resolve prefix to namespace without a stylesheet, syntax error.
+    return m_styleSheet->namespaceURIFromPrefix(prefix);
 }
 
 void CSSSelectorParser::prependTypeSelectorIfNeeded(const AtomicString& namespacePrefix, const AtomicString& elementName, CSSParserSelector* compoundSelector)