Correctly set 'firstPartyForCookies' in 'RenderFrameImpl::willSendRequest'.

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 b8bb6474ab3db3ce89ac6d5e0c8cbe212ba90fe6..a08db328176a22470e25704cd1c56df2816fda84 100644
--- a/third_party/WebKit/Source/core/dom/Document.cpp
+++ b/third_party/WebKit/Source/core/dom/Document.cpp
@@ -4164,6 +4164,10 @@ String Document::lastModified() const
 
 const KURL Document::firstPartyForCookies() const
 {
+    // If this is an imported document, grab its master document's first-party:
+    if (importsController() && importsController()->master() && importsController()->master() != this)
+        return importsController()->master()->firstPartyForCookies();
+
     // TODO(mkwst): This doesn't correctly handle sandboxed documents; we want to look at their URL,
     // but we can't because we don't know what it is.
     Frame* top = frame()->tree().top();