Enable template checking for the clang plugin on Mac.

Enable template checking for the clang plugin on Mac.

This enables the plugin to check templated classes for the following
things:
- virtual functions have {virtual,override,final} correctly specified.
- Refcounted types have private destructors.
- WeakPtrFactory members are the last members

BUG=441916
Committed: https://crrev.com/dcf757517b8b95793c0aff1509cb356cdcbd59e9
Cr-Commit-Position: refs/heads/master@{#331743}

[dcheng, 2015-05-27 23:45:31]

dcheng@chromium.org changed reviewers:
+ thakis@chromium.org

[dcheng, 2015-05-27 23:56:39]

It might work. It might not work. One way to find out!

(I'm building locally on my Mac, but there's still 5k build steps left and I'm
too impatient to watch it any longer)

[Nico, 2015-05-28 02:35:35]

lgtm

[dcheng, 2015-05-28 03:29:27]

The CQ bit was checked by dcheng@chromium.org

[commit-bot: I haz the power, 2015-05-28 03:30:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1160033002/1

[commit-bot: I haz the power, 2015-05-28 05:21:46]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-28 05:22:39]

Patchset 1 (id:??) landed as
https://crrev.com/dcf757517b8b95793c0aff1509cb356cdcbd59e9
Cr-Commit-Position: refs/heads/master@{#331743}