Ancestors count towards first-partyness.

Source/core/dom/Document.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 3961 matching lines @@
     // FIXME: If this document came from the file system, the HTML5
     // specificiation tells us to read the last modification date from the file
     // system.
     if (!foundDate)
         date.setMillisecondsSinceEpochForDateTime(convertToLocalTime(currentTimeMS()));
     return String::format("%02d/%02d/%04d %02d:%02d:%02d", date.month() + 1, date.monthDay(), date.fullYear(), date.hour(), date.minute(), date.second());
 }
 
 const KURL& Document::firstPartyForCookies() const
 {
+    if (!RuntimeEnabledFeatures::firstPartyIncludesAncestorsEnabled())
+        return topDocument().url();
+
+    // We're intentionally using the URL of each document rather than the document's SecurityOrigin.
+    // Sandboxing a document into a unique origin shouldn't effect first-/third-party status for
+    // cookies and site data.
+    RefPtr<SecurityOrigin> topOrigin = SecurityOrigin::create(topDocument().url());
+    const Document* currentDocument = this;
+    while (currentDocument) {
+        // Skip over srcdoc documents, as they are always same-origin with their closest non-srcdoc parent.
+        while (currentDocument->isSrcdocDocument())
+            currentDocument = currentDocument->parentDocument();
+        ASSERT(currentDocument);
+
+        if (!topOrigin->canRequest(currentDocument->url()))
+            return SecurityOrigin::urlWithUniqueSecurityOrigin();
+
+        currentDocument = currentDocument->parentDocument();
+    }
+
     return topDocument().url();
 }
 
 static bool isValidNameNonASCII(const LChar* characters, unsigned length)
 {
     if (!isValidNameStart(characters[0]))
         return false;
 
     for (unsigned i = 1; i < length; ++i) {
         if (!isValidNamePart(characters[i]))
@@ skipping 1708 matching lines @@
 #ifndef NDEBUG
 using namespace blink;
 void showLiveDocumentInstances()
 {
     WeakDocumentSet& set = liveDocumentSet();
     fprintf(stderr, "There are %u documents currently alive:\n", set.size());
     for (Document* document : set)
         fprintf(stderr, "- Document %p URL: %s\n", document, document->url().string().utf8().data());
 }
 #endif