Remove "nacl_defines" from the GN build.

Remove "nacl_defines" from the GN build.

The following NaCl-related defines are now provided by
native_client/src/include/build_config.h instead of the build system:
  NACL_WINDOWS
  NACL_LINUX
  NACL_OSX
  NACL_ANDROID
  NACL_BUILD_ARCH
  NACL_BUILD_SUBARCH

This change removes these definitions from gn(i) files, as they no longer need
to be provided by the build system.

BUG=440012
Committed: https://crrev.com/2b59c805e1e097e10c36650573fa5c47cd47dc03
Cr-Commit-Position: refs/heads/master@{#315408}

[teravest, 2015-02-09 22:05:40]

teravest@chromium.org changed reviewers:
+ mseaborn@chromium.org

[Mark Seaborn, 2015-02-09 22:14:14]

LGTM

[teravest, 2015-02-09 22:17:02]

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

[teravest, 2015-02-09 22:17:02]

+thakis for chrome/browser/

[Nico, 2015-02-09 22:17:57]

lgtm

[teravest, 2015-02-09 22:21:23]

The CQ bit was checked by teravest@chromium.org

[commit-bot: I haz the power, 2015-02-09 22:22:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/908003002/1

[commit-bot: I haz the power, 2015-02-09 22:26:12]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-09 22:26:58]

Patchset 1 (id:??) landed as
https://crrev.com/2b59c805e1e097e10c36650573fa5c47cd47dc03
Cr-Commit-Position: refs/heads/master@{#315408}