[PPAPI] Make the NaCl SDK presubmit check a warning instead of an error.

[PPAPI] Make the NaCl SDK presubmit check a warning instead of an error.

There are some PPAPI dev interfaces that should not be added to the SDK, so it
is better to warn instead of making it a presubmit error.

BUG=none
R=teravest@chromium.org
TEST=made change to ppapi/cpp/buffer_dev.h, ran git cl presubmit

[teravest, 2014-07-09 20:55:38]

lgtm

[binji, 2014-07-09 20:57:24]

The CQ bit was checked by binji@chromium.org

[commit-bot: I haz the power, 2014-07-09 20:59:22]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/binji@chromium.org/379923010/1

[commit-bot: I haz the power, 2014-07-10 00:35:20]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  mac_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/mac_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-07-10 02:58:40]

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

[commit-bot: I haz the power, 2014-07-10 02:58:41]

Try jobs failed on following builders:
  mac_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/mac_chromium_rel/buil...)