[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 939444bee8c432fa27bd701d459773f7f4514984..7d4077ce7996c77c4e06f73aa4dc2ac01ffee8ea 100644
--- a/mojo/edk/system/broker_messages.h
+++ b/mojo/edk/system/broker_messages.h
@@ -19,35 +19,41 @@ namespace edk {
 // This header defines the message format between ChildBroker and
 // ChildBrokerHost.
 
-#if defined(OS_WIN)
-// Windows only messages needed because sandboxed child processes need the
-// parent's help. They are sent synchronously from child to parent and each have
+// Sandbox processes need the parent's help to create shared buffers.
+// They are sent synchronously from child to parent and each have
 // a response. They are sent over a raw pipe.
-enum WindowsSandboxMessages : uint32_t {
+enum SandboxMessages : uint32_t {
+#if defined(OS_WIN)
   // The reply is two HANDLEs.
   CREATE_PLATFORM_CHANNEL_PAIR = 0,
   // The reply is tokens of the same count of passed in handles.
   HANDLE_TO_TOKEN,
   // The reply is handles of the same count of passed in tokens.
   TOKEN_TO_HANDLE,
+#else
+  // The reply is a PlatformHandle.
+  CREATE_SHARED_BUFFER = 0,
+#endif
 };
 
 // Definitions of the raw bytes sent in messages.
 
 struct BrokerMessage {
   uint32_t size;
-  WindowsSandboxMessages id;
+  SandboxMessages id;
+
+#if defined(OS_WIN)
   // Data, if any, follows.
   union {
     HANDLE handles[1];  // If HANDLE_TO_TOKEN.
     uint64_t tokens[1];  // If TOKEN_TO_HANDLE.
   };
+#else
+  uint32_t shared_buffer_size;  // Size of the shared buffer to create.
+#endif
 };
 
-const int kBrokerMessageHeaderSize =
-  sizeof(uint32_t) + sizeof(WindowsSandboxMessages);
-
-#endif
+const int kBrokerMessageHeaderSize = sizeof(uint32_t) + sizeof(SandboxMessages);
 
 // Route id used for messages between ChildBroker and ChildBrokerHost.
 const uint64_t kBrokerRouteId = 1;