[mojo] Add CreateSharedBuffer method to Broker.

mojo/edk/system/broker_state.cc

Index: mojo/edk/system/broker_state.cc
diff --git a/mojo/edk/system/broker_state.cc b/mojo/edk/system/broker_state.cc
index 90ea00dbc1a526568a8ee23d7ea5ff35ba750373..532843dc4c613a76292fc9d7f74a065844190825 100644
--- a/mojo/edk/system/broker_state.cc
+++ b/mojo/edk/system/broker_state.cc
@@ -11,6 +11,8 @@
 #include "base/rand_util.h"
 #include "mojo/edk/embedder/embedder_internal.h"
 #include "mojo/edk/embedder/platform_channel_pair.h"
+#include "mojo/edk/embedder/platform_shared_buffer.h"
+#include "mojo/edk/embedder/platform_support.h"
 #include "mojo/edk/system/child_broker_host.h"
 #include "mojo/edk/system/message_pipe_dispatcher.h"
 #include "mojo/edk/system/routed_raw_channel.h"
@@ -65,6 +67,11 @@ void BrokerState::TokenToHandle(const uint64_t* tokens,
     }
   }
 }
+#else
+scoped_refptr<PlatformSharedBuffer> BrokerState::CreateSharedBuffer(
+    size_t num_bytes) {
+  return internal::g_platform_support->CreateSharedBuffer(num_bytes);
+}
 #endif
 
 void BrokerState::ConnectMessagePipe(uint64_t pipe_id,