GN: Fix nacl_defines used in chrome/browser

GN: Fix nacl_defines used in chrome/browser

chrome/browser/ was using a nacl_defines set from //native_client/...,
which is not the same as the nacl_defines used by the gyp build in
//components/nacl/. In particular, defining _XOPEN_SOURCE=600 breaks
compilation on mac in highly mysterious ways. This adds a
//components/nacl/nacl_defines.gni that mirros the defines in
//components/nacl/nacl_defines.gypi to bring the builds back in line
with each other.

R=brettw@chromium.org
TBR=noelallen@chromium.org for components/nacl

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=286322

[brettw, 2014-07-29 20:37:51]

lgtm

[jamesr, 2014-07-29 22:13:30]

The CQ bit was checked by jamesr@chromium.org

[commit-bot: I haz the power, 2014-07-29 22:16:33]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jamesr@chromium.org/424043002/1

[commit-bot: I haz the power, 2014-07-29 22:43:58]

Change committed as 286322