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

mojo/system/local_data_pipe.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_SYSTEM_LOCAL_DATA_PIPE_H_
 #define MOJO_SYSTEM_LOCAL_DATA_PIPE_H_
 
 #include "base/basictypes.h"
 #include "base/memory/aligned_memory.h"
 #include "base/memory/ref_counted.h"
@@ skipping 47 matching lines @@
   virtual MojoWaitFlags ConsumerSatisfiableFlagsNoLock() OVERRIDE;
 
   void EnsureBufferNoLock();
   void DestroyBufferNoLock();
 
   // Get the maximum (single) write/read size right now (in number of elements);
   // result fits in a |uint32_t|.
   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.
   size_t start_index_;
   size_t current_num_bytes_;
 
   DISALLOW_COPY_AND_ASSIGN(LocalDataPipe);
 };
 
 }  // namespace system
 }  // namespace mojo
 
 #endif  // MOJO_SYSTEM_LOCAL_DATA_PIPE_H_