Changes to third_party/cython build rules to allow source sets for

Changes to third_party/cython build rules to allow source sets for
python modules composed of C++ and/or Cython files.

This change is necessary for https://codereview.chromium.org/792843002/.
It is also backward-incompatible with the current mojo/public/python
build rules; I change them in the same changelist to avoid the
dependency issue. Once this CL lands, I will roll the changes into the
mojo repository from chromium.

R=qsr@chromium.org,jamesr@chromium.org

Committed: https://crrev.com/560d48f40a669bfaf06a830d0cefb926628b0ed1
Cr-Commit-Position: refs/heads/master@{#308331}

[qsr, 2014-12-12 16:50:02]

lgtm

[jamesr, 2014-12-13 01:47:16]

mojo/ lgtm

I don't really know anything about cython

[etiennej, 2014-12-15 08:59:15]

The CQ bit was checked by etiennej@chromium.org

[commit-bot: I haz the power, 2014-12-15 08:59:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/799923003/1

[commit-bot: I haz the power, 2014-12-15 09:50:47]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-15 09:51:36]

Patchset 1 (id:??) landed as
https://crrev.com/560d48f40a669bfaf06a830d0cefb926628b0ed1
Cr-Commit-Position: refs/heads/master@{#308331}

[etiennej, 2015-01-08 13:32:04]

Patchset #2 (id:20001) has been deleted