--- a/dom/base/PostMessageEvent.cpp
+++ b/dom/base/PostMessageEvent.cpp
@@ -21,17 +21,16 @@
 namespace mozilla {
 namespace dom {
 
 namespace {
 
 struct StructuredCloneInfo
 {
   PostMessageEvent* event;
-  bool subsumes;
   nsPIDOMWindow* window;
 
   // This hashtable contains the transferred ports - used to avoid duplicates.
   nsTArray<nsRefPtr<MessagePortBase>> transferredPorts;
 
   // This array is populated when the ports are cloned.
   nsTArray<nsRefPtr<MessagePortBase>> clonedPorts;
 };
@@ -112,17 +111,17 @@ PostMessageEvent::WriteStructuredClone(J
                                        void *closure)
 {
   StructuredCloneInfo* scInfo = static_cast<StructuredCloneInfo*>(closure);
   NS_ASSERTION(scInfo, "Must have scInfo!");
 
   // See if this is a File/Blob object.
   {
     Blob* blob = nullptr;
-    if (scInfo->subsumes && NS_SUCCEEDED(UNWRAP_OBJECT(Blob, obj, blob))) {
+    if (NS_SUCCEEDED(UNWRAP_OBJECT(Blob, obj, blob))) {
       BlobImpl* blobImpl = blob->Impl();
       if (JS_WriteUint32Pair(writer, SCTAG_DOM_BLOB, 0) &&
           JS_WriteBytes(writer, &blobImpl, sizeof(blobImpl))) {
         scInfo->event->StoreISupports(blobImpl);
         return true;
       }
     }
   }
@@ -130,17 +129,17 @@ PostMessageEvent::WriteStructuredClone(J
   nsCOMPtr<nsIXPConnectWrappedNative> wrappedNative;
   nsContentUtils::XPConnect()->
     GetWrappedNativeOfJSObject(cx, obj, getter_AddRefs(wrappedNative));
   if (wrappedNative) {
     uint32_t scTag = 0;
     nsISupports* supports = wrappedNative->Native();
 
     nsCOMPtr<nsIDOMFileList> list = do_QueryInterface(supports);
-    if (list && scInfo->subsumes)
+    if (list)
       scTag = SCTAG_DOM_FILELIST;
 
     if (scTag)
       return JS_WriteUint32Pair(writer, scTag, 0) &&
              JS_WriteBytes(writer, &supports, sizeof(supports)) &&
              scInfo->event->StoreISupports(supports);
   }
 
@@ -374,24 +373,22 @@ PostMessageEvent::Run()
                             internalEvent,
                             static_cast<dom::Event*>(event.get()),
                             &status);
   return NS_OK;
 }
 
 bool
 PostMessageEvent::Write(JSContext* aCx, JS::Handle<JS::Value> aMessage,
-                        JS::Handle<JS::Value> aTransfer, bool aSubsumes,
-                        nsPIDOMWindow* aWindow)
+                        JS::Handle<JS::Value> aTransfer, nsPIDOMWindow* aWindow)
 {
   // We *must* clone the data here, or the JS::Value could be modified
   // by script
   StructuredCloneInfo scInfo;
   scInfo.event = this;
   scInfo.window = aWindow;
-  scInfo.subsumes = aSubsumes;
 
   return mBuffer.write(aCx, aMessage, aTransfer, &sPostMessageCallbacks,
                        &scInfo);
 }
 
 } // namespace dom
 } // namespace mozilla

--- a/dom/base/PostMessageEvent.h
+++ b/dom/base/PostMessageEvent.h
@@ -34,18 +34,17 @@ public:
 
   PostMessageEvent(nsGlobalWindow* aSource,
                    const nsAString& aCallerOrigin,
                    nsGlobalWindow* aTargetWindow,
                    nsIPrincipal* aProvidedPrincipal,
                    bool aTrustedCaller);
 
   bool Write(JSContext* aCx, JS::Handle<JS::Value> aMessage,
-             JS::Handle<JS::Value> aTransfer, bool aSubsumes,
-             nsPIDOMWindow* aWindow);
+             JS::Handle<JS::Value> aTransfer, nsPIDOMWindow* aWindow);
 
 private:
   ~PostMessageEvent();
 
   const MessagePortIdentifier& GetPortIdentifier(uint64_t aId);
 
   MessagePortIdentifier* NewPortIdentifier(uint64_t* aPosition);
 

--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -8556,23 +8556,20 @@ nsGlobalWindow::PostMessageMozOuter(JSCo
     new PostMessageEvent(nsContentUtils::IsCallerChrome() || !callerInnerWin
                          ? nullptr
                          : callerInnerWin->GetOuterWindowInternal(),
                          origin,
                          this,
                          providedPrincipal,
                          nsContentUtils::IsCallerChrome());
 
-  nsIPrincipal* principal = GetPrincipal();
   JS::Rooted<JS::Value> message(aCx, aMessage);
   JS::Rooted<JS::Value> transfer(aCx, aTransfer);
-  bool subsumes;
-
-  if (NS_FAILED(callerPrin->Subsumes(principal, &subsumes)) ||
-      !event->Write(aCx, message, transfer, subsumes, this)) {
+
+  if (!event->Write(aCx, message, transfer, this)) {
     aError.Throw(NS_ERROR_DOM_DATA_CLONE_ERR);
     return;
   }
 
   aError = NS_DispatchToCurrentThread(event);
 }
 
 void