Revert of mojo: fix python unittests. (patchset #1 id:1 of...

Revert of mojo: fix python unittests. (patchset #1 id:1 of https://codereview.chromium.org/586683002/)

Reason for revert:
Linux Tests mojo_python_bindings_tests failure
http://build.chromium.org/p/chromium.linux/builders/Linux%20Tests/builds/13712

======================================================================
ERROR: testDefaultsTestSerializationDeserialization (bindings_serialization_deserialization_unittest.SerializationDeserializationTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/mnt/data/b/build/slave/Linux_Tests/build/src/mojo/python/tests/bindings_serialization_deserialization_unittest.py", line 125, in testDefaultsTestSerializationDeserialization
    v2 = sample_service_mojom.DefaultsTest.Deserialize(data, handles)
  File "/mnt/data/b/build/slave/Linux_Tests/build/src/out/Release/python/mojo/bindings/reflection.py", line 113, in Deserialize
    serialization_object.Deserialize(fields, data, handles)
  File "/mnt/data/b/build/slave/Linux_Tests/build/src/out/Release/python/mojo/bindings/serialization.py", line 82, in Deserialize
    (_, version) = HEADER_STRUCT.unpack_from(data)
TypeError: unpack_from() argument 1 must be string or read-only buffer, not bytearray

Original issue's description:
> mojo: fix python unittests.
>
> TBR=sdefresne@chromium.org
>
> Committed: https://chromium.googlesource.com/chromium/src/+/80ced42d624a1eec8e8a2f0c0c760ab576d65ead

TBR=sdefresne@chromium.org,qsr@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/587653003

Cr-Commit-Position: refs/heads/master@{#295688}

mojo/public/python/mojo/bindings/descriptor.py

@@ -205,7 +205,7 @@ class StringType(PointerType):
   def DeserializePointer(self, size, nb_elements, data, handles):
     string_array = self._array_type.DeserializeArray(
         size, nb_elements, data, handles)
-    return unicode(string_array.tostring(), 'utf8')
+    return unicode(buffer(string_array), 'utf8')
 
 
 class HandleType(SerializableType):