Mojo: DataPipe: Implement "may discard" for two-phase writes.

mojo/system/local_data_pipe.h

Index: mojo/system/local_data_pipe.h
diff --git a/mojo/system/local_data_pipe.h b/mojo/system/local_data_pipe.h
index f9c0aae1f75b634e5680c47c33a486c4edfa5fd5..97d108e4bdd71a32ddd643b4b9e35b08f1f1814a 100644
--- a/mojo/system/local_data_pipe.h
+++ b/mojo/system/local_data_pipe.h
@@ -65,6 +65,10 @@ class MOJO_SYSTEM_IMPL_EXPORT LocalDataPipe : public DataPipe {
   size_t GetMaxNumBytesToWriteNoLock();
   size_t GetMaxNumBytesToReadNoLock();
 
+  // Marks the given number of bytes as consumed/discarded. |num_bytes| must be
+  // greater than |current_num_bytes_|.
+  void MarkDataAsConsumedNoLock(size_t num_bytes);
+
   // The members below are protected by |DataPipe|'s |lock_|:
   scoped_ptr_malloc<char, base::ScopedPtrAlignedFree> buffer_;
   // Circular buffer.