Make Python not support "all or none" for two-phase data pipe read/write.

mojo/python/tests/system_unittest.py

 # Copyright 2014 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.
 
 import random
 import time
 
 import mojo_unittest
 
 # pylint: disable=E0611
@@ skipping 246 matching lines @@
     self.assertEquals((system.RESULT_OK, DATA_SIZE),
                       pipes.producer_handle.WriteData(data))
     self.assertEquals((system.RESULT_OK, data),
                       pipes.consumer_handle.ReadData(
                           bytearray(DATA_SIZE), system.READ_DATA_FLAG_PEEK))
     self.assertEquals((system.RESULT_OK, data),
                       pipes.consumer_handle.ReadData(bytearray(DATA_SIZE)))
 
   def testTwoPhaseWriteOnDataPipe(self):
     pipes = system.DataPipe()
-    (res, buf) = pipes.producer_handle.BeginWriteData(DATA_SIZE)
+    (res, buf) = pipes.producer_handle.BeginWriteData()
     self.assertEquals(system.RESULT_OK, res)
     self.assertGreaterEqual(len(buf.buffer), DATA_SIZE)
     data = _GetRandomBuffer(DATA_SIZE)
     buf.buffer[0:DATA_SIZE] = data
     self.assertEquals(system.RESULT_OK, buf.End(DATA_SIZE))
     self.assertEquals((system.RESULT_OK, data),
                       pipes.consumer_handle.ReadData(bytearray(DATA_SIZE)))
 
   def testTwoPhaseReadOnDataPipe(self):
     pipes = system.DataPipe()
@@ skipping 40 matching lines @@
     self.assertEquals(system.RESULT_OK, res1)
     self.assertEquals(system.RESULT_OK, res2)
     self.assertEquals(DATA_SIZE, len(buf1.buffer))
     self.assertEquals(DATA_SIZE, len(buf2.buffer))
     self.assertEquals(buf1.buffer, buf2.buffer)
 
     buf1.buffer[:] = data
     self.assertEquals(data, buf1.buffer)
     self.assertEquals(data, buf2.buffer)
     self.assertEquals(buf1.buffer, buf2.buffer)