Remove native_client_sdk from the PNG presubmit check

Remove native_client_sdk from the PNG presubmit check

The native_client_sdk directory contains auto-generated documentation that
includes PNGs that are flagged by the presubmit check. These PNGs are not used
by Chrome, so they should be ignored by the check.

BUG=150046
R=phajdan.jr@chromium.org

Committed: https://crrev.com/0dcdf34d61c72798ad8949882efa2a42bf391cf3
Cr-Commit-Position: refs/heads/master@{#308125}

[Paweł Hajdan Jr., 2014-12-12 10:34:30]

LGTM

[binji, 2014-12-12 17:55:02]

The CQ bit was checked by binji@chromium.org

[commit-bot: I haz the power, 2014-12-12 17:56:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/802463002/20001

[commit-bot: I haz the power, 2014-12-12 18:32:40]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-12-12 18:33:54]

Patchset 2 (id:??) landed as
https://crrev.com/0dcdf34d61c72798ad8949882efa2a42bf391cf3
Cr-Commit-Position: refs/heads/master@{#308125}