Fix GN mac toolchain error.

Fix GN mac toolchain error.

When you run `gn gen out-gn/Debug` on Mac you get the following:

ERROR at //build/toolchain/mac/BUILD.gn:200:12: Build argument has no effect.
      os = current_os
           ^---------
The variable "os" was set as a build argument
but never appeared in a declare_args() block in any buildfile.

BUG=None
TEST=`gn gen out-gn/Debug` passes without the above error
R=dpranke@chromium.org

Committed: https://crrev.com/53ec706bbc5debe052dd51e4d93e2de4c2802ab2
Cr-Commit-Position: refs/heads/master@{#324109}

[tfarina, 2015-04-07 18:03:13]

https://codereview.chromium.org/1064063002/diff/1/build/toolchain/mac/BUILD.gn
File build/toolchain/mac/BUILD.gn (right):

https://codereview.chromium.org/1064063002/diff/1/build/toolchain/mac/BUILD.g...
build/toolchain/mac/BUILD.gn:199: target_os = current_os
is this correct? should be something else like host_os?

[Dirk Pranke, 2015-04-07 18:48:18]

https://codereview.chromium.org/1064063002/diff/1/build/toolchain/mac/BUILD.gn
File build/toolchain/mac/BUILD.gn (right):

https://codereview.chromium.org/1064063002/diff/1/build/toolchain/mac/BUILD.g...
build/toolchain/mac/BUILD.gn:199: target_os = current_os
On 2015/04/07 18:03:13, tfarina wrote:
> is this correct? should be something else like host_os?

No, definitely not. You should never change target_os or host_os.

In this case, os is just left over, and can be deleted. I don't think anything
in the tree references it.

[tfarina, 2015-04-07 20:06:34]

https://codereview.chromium.org/1064063002/diff/1/build/toolchain/mac/BUILD.gn
File build/toolchain/mac/BUILD.gn (right):

https://codereview.chromium.org/1064063002/diff/1/build/toolchain/mac/BUILD.g...
build/toolchain/mac/BUILD.gn:199: target_os = current_os
On 2015/04/07 18:48:18, Dirk Pranke wrote:
> On 2015/04/07 18:03:13, tfarina wrote:
> > is this correct? should be something else like host_os?
> 
> No, definitely not. You should never change target_os or host_os.
> 
> In this case, os is just left over, and can be deleted. I don't think anything
> in the tree references it.
> 

Done.

[Dirk Pranke, 2015-04-07 20:17:16]

lgtm

[tfarina, 2015-04-07 20:33:16]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-04-07 20:33:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1064063002/20001

[commit-bot: I haz the power, 2015-04-07 20:38:22]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-07 20:39:16]

Patchset 2 (id:??) landed as
https://crrev.com/53ec706bbc5debe052dd51e4d93e2de4c2802ab2
Cr-Commit-Position: refs/heads/master@{#324109}