document.lastModified: treat invalid dates like unknown ones.

third_party/WebKit/Source/core/dom/Document.cpp

Index: third_party/WebKit/Source/core/dom/Document.cpp
diff --git a/third_party/WebKit/Source/core/dom/Document.cpp b/third_party/WebKit/Source/core/dom/Document.cpp
index dc5974ba26fa9533ccc3859d07c7794ee1d1ad18..c621e248b7dd1e6395708ac2e5537879aef650a7 100644
--- a/third_party/WebKit/Source/core/dom/Document.cpp
+++ b/third_party/WebKit/Source/core/dom/Document.cpp
@@ -4666,9 +4666,12 @@ String Document::lastModified() const {
       const AtomicString& httpLastModified =
           documentLoader->response().httpHeaderField(HTTPNames::Last_Modified);
       if (!httpLastModified.isEmpty()) {
-        date.setMillisecondsSinceEpochForDateTime(
-            convertToLocalTime(parseDate(httpLastModified)));
-        foundDate = true;
+        double dateValue = parseDate(httpLastModified);
+        if (!std::isnan(dateValue)) {
+          date.setMillisecondsSinceEpochForDateTime(
+              convertToLocalTime(dateValue));
+          foundDate = true;
+        }
       }
     }
   }