ipc: Make a new class PlaceholderBrokerableAttachment.

ipc/brokerable_attachment.h

Index: ipc/brokerable_attachment.h
diff --git a/ipc/brokerable_attachment.h b/ipc/brokerable_attachment.h
index 9c2bbbd69ec444ac0770a1df93cf7ae34110be20..e5a701759e8991b2d9f931cba4876a29b75f54f0 100644
--- a/ipc/brokerable_attachment.h
+++ b/ipc/brokerable_attachment.h
@@ -51,6 +51,7 @@ class IPC_EXPORT BrokerableAttachment : public MessageAttachment {
   };
 
   enum BrokerableType {
+    PLACEHOLDER,
     WIN_HANDLE,
   };
 
@@ -61,31 +62,26 @@ class IPC_EXPORT BrokerableAttachment : public MessageAttachment {
   // can be used.
   bool NeedsBrokering() const;
 
-  // Fills in the data of this instance with the data from |attachment|.
-  // This instance must require brokering, |attachment| must be brokered, and
-  // both instances must have the same identifier.
-  virtual void PopulateWithAttachment(
-      const BrokerableAttachment* attachment) = 0;
-
   // Returns TYPE_BROKERABLE_ATTACHMENT
   Type GetType() const override;
 
   virtual BrokerableType GetBrokerableType() const = 0;
 
+// MessageAttachment override.
+#if defined(OS_POSIX)
+  base::PlatformFile TakePlatformFile() override;
+#endif  // OS_POSIX
+
  protected:
   BrokerableAttachment();
-  BrokerableAttachment(const AttachmentId& id, bool needs_brokering);
+  BrokerableAttachment(const AttachmentId& id);
   ~BrokerableAttachment() override;
 
-  void SetNeedsBrokering(bool needs_brokering);
-
  private:
   // This member uniquely identifies a BrokerableAttachment across all Chrome
   // processes.
   const AttachmentId id_;
 
-  // Whether the attachment still needs to be filled in by an AttachmentBroker.
-  bool needs_brokering_;
   DISALLOW_COPY_AND_ASSIGN(BrokerableAttachment);
 };