Update HTMLElement.dir / Document.dir to return only known values

Source/core/html/HTMLElement.cpp

Index: Source/core/html/HTMLElement.cpp
diff --git a/Source/core/html/HTMLElement.cpp b/Source/core/html/HTMLElement.cpp
index 1c451cff4c3b76e7a0880e86a8f605642aaca74b..7fae6b26659288df365a9880d03686660af221b9 100644
--- a/Source/core/html/HTMLElement.cpp
+++ b/Source/core/html/HTMLElement.cpp
@@ -629,6 +629,35 @@ void HTMLElement::setTranslate(bool enable)
     setAttribute(translateAttr, enable ? "yes" : "no");
 }
 
+// Returns the conforming 'dir' value associated with the state the attribute is in (in its canonical case), if any,
+// or the empty string if the attribute is in a state that has no associated keyword value or if the attribute is
+// not in a defined state (e.g. the attribute is missing and there is no missing value default).
+// http://www.whatwg.org/specs/web-apps/current-work/multipage/common-dom-interfaces.html#limited-to-only-known-values
+static inline const AtomicString& toValidDirValue(const AtomicString& value)
+{
+    DEFINE_STATIC_LOCAL(const AtomicString, ltrValue, ("ltr", AtomicString::ConstructFromLiteral));
+    DEFINE_STATIC_LOCAL(const AtomicString, rtlValue, ("rtl", AtomicString::ConstructFromLiteral));
+    DEFINE_STATIC_LOCAL(const AtomicString, autoValue, ("auto", AtomicString::ConstructFromLiteral));
+
+    if (equalIgnoringCase(value, ltrValue))
+        return ltrValue;
+    if (equalIgnoringCase(value, rtlValue))
+        return rtlValue;
+    if (equalIgnoringCase(value, autoValue))
+        return autoValue;
+    return nullAtom;
+}
+
+const AtomicString& HTMLElement::dir()
+{
+    return toValidDirValue(fastGetAttribute(dirAttr));
+}
+
+void HTMLElement::setDir(const AtomicString& value)
+{
+    setAttribute(dirAttr, value);
+}
+
 HTMLFormElement* HTMLElement::findFormAncestor() const
 {
     return Traversal<HTMLFormElement>::firstAncestor(*this);