[mojo] Add CreateSharedBuffer method to Broker.

mojo/edk/system/broker_messages.h

Index: mojo/edk/system/broker_messages.h
diff --git a/mojo/edk/system/broker_messages.h b/mojo/edk/system/broker_messages.h
index 0ce0960d685b9fadc8e0cb5c1c0c7adf936bfd2c..f7616c13d0782b118e8cb1a2662878514f954a6c 100644
--- a/mojo/edk/system/broker_messages.h
+++ b/mojo/edk/system/broker_messages.h
@@ -29,21 +29,37 @@ enum WindowsSandboxMessages {
   TOKEN_TO_HANDLE,
 };
 
+#else
+// Sandbox processes need the parent's help to create shared buffers.
+enum PosixSandboxMessages {
+  // The reply is a PlatformHandle.
+  CREATE_SHARED_BUFFER = 0
+};
+#endif
+
 // Definitions of the raw bytes sent in messages.
 
 struct BrokerMessage {
   uint32_t size;
+#if defined(OS_WIN)
   WindowsSandboxMessages id;
   // Data, if any, follows.
   union {
     HANDLE handles[1];  // If HANDLE_TO_TOKEN.
     uint64_t tokens[1];  // If TOKEN_TO_HANDLE.
   };
-};
+#else
+  PosixSandboxMessages id;
 
-const int kBrokerMessageHeaderSize =
-  sizeof(uint32_t) + sizeof(WindowsSandboxMessages);
+  uint32_t num_bytes;  // Size of the shared buffer to create.
+#endif
+};
 
+const int kBrokerMessageHeaderSize = sizeof(uint32_t) +
+#if defined(OS_WIN)
+                                     sizeof(WindowsSandboxMessages);
+#else
+                                     sizeof(PosixSandboxMessages);
 #endif
 
 // Route id used for messages between ChildBroker and ChildBrokerHost.