Mojo: Add signal for new data pipe consumer data

mojo/edk/system/data_pipe_consumer_dispatcher.h

Index: mojo/edk/system/data_pipe_consumer_dispatcher.h
diff --git a/mojo/edk/system/data_pipe_consumer_dispatcher.h b/mojo/edk/system/data_pipe_consumer_dispatcher.h
index 8239b11462f9851793790928ef693eabdd02cbac..b323c16d83f3bcfaa930b981b1824b205061b279 100644
--- a/mojo/edk/system/data_pipe_consumer_dispatcher.h
+++ b/mojo/edk/system/data_pipe_consumer_dispatcher.h
@@ -117,6 +117,9 @@ class MOJO_SYSTEM_IMPL_EXPORT DataPipeConsumerDispatcher final
   uint32_t read_offset_ = 0;
   uint32_t bytes_available_ = 0;
 
+  // Indicates whether any new data is available since the last read attempt.
+  bool new_data_available_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(DataPipeConsumerDispatcher);
 };