mojo: Add deserialization to python structs.

mojo: Add deserialization to python structs.

This is a reland of https://codereview.chromium.org/578263003 using
buffer instead of memoryview, because on some early version of python
2.7, memoryview cannot be used with struct.unpack_from.

BUG=415491
R=sdefresne@chromium.org,cmasone@chromium.org

Committed: https://crrev.com/37dd5f484593b1c8b92b825a9648edf7fb6944e6
Cr-Commit-Position: refs/heads/master@{#295684}

[qsr, 2014-09-19 08:29:36]

The patch 1 is the original CL.

[sdefresne, 2014-09-19 08:32:10]

lgtm

[qsr, 2014-09-19 08:54:27]

The CQ bit was checked by qsr@chromium.org

[commit-bot: I haz the power, 2014-09-19 08:55:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/588493002/40001

[commit-bot: I haz the power, 2014-09-19 09:49:43]

Committed patchset #3 (id:40001) as 60fe04bc91783a29a4c3337ff79feead85d648e4

[commit-bot: I haz the power, 2014-09-19 09:50:27]

Patchset 3 (id:??) landed as
https://crrev.com/37dd5f484593b1c8b92b825a9648edf7fb6944e6
Cr-Commit-Position: refs/heads/master@{#295684}

[vasilii, 2014-09-19 12:24:29]

A revert of this CL (patchset #3 id:40001) has been created in
https://codereview.chromium.org/584003002/ by vasilii@chromium.org.

The reason for reverting is: 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.