Disable implicit conversions of scoped_refptr to T* on Mac.

Disable implicit conversions of scoped_refptr to T* on Mac.

The implicit conversion has already been disabled for Linux and ChromeOS
builds. https://codereview.chromium.org/510323002 explains the rationale
for proceeding incrementally.

BUG=110610
TBR=thakis@chromium.org

Committed: https://crrev.com/ed3e65c71b7566004fc75da2799f1f9e7e4b299c
Cr-Commit-Position: refs/heads/master@{#295889}

[dcheng, 2014-09-20 11:12:16]

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

[dcheng, 2014-09-20 18:31:36]

The CQ bit was checked by dcheng@chromium.org

[commit-bot: I haz the power, 2014-09-20 18:32:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/589853002/1

[commit-bot: I haz the power, 2014-09-20 20:08:58]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-09-20 20:08:58]

Try jobs failed on following builders:
  mac_chromium_rel_swarming on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[dcheng, 2014-09-20 20:09:37]

The CQ bit was checked by dcheng@chromium.org

[commit-bot: I haz the power, 2014-09-20 20:10:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/589853002/1

[commit-bot: I haz the power, 2014-09-20 20:53:40]

Committed patchset #1 (id:1) as 4bbd71d39ac6446ba8a2e9e6213594b074d9c7b3

[commit-bot: I haz the power, 2014-09-20 20:54:15]

Patchset 1 (id:??) landed as
https://crrev.com/ed3e65c71b7566004fc75da2799f1f9e7e4b299c
Cr-Commit-Position: refs/heads/master@{#295889}

[Nico, 2014-09-20 22:29:48]

lgtm

Nice!!