Add GN Build file for rtc_sound target.

Add GN Build file for rtc_sound target.

Tested on Linux with the following command lines:

$ gn gen out-gn/Release --args='is_debug=false target_cpu="x64"
build_with_chromium=false'
$ ninja -C out-gn/Release rtc_sound

BUG=webrtc:4160
R=kjellander@webrtc.org

Committed: https://crrev.com/f054819e257a4f9cbb7fa82ba51dc2335f4359ec
Cr-Commit-Position: refs/heads/master@{#10405}

[kjellander_webrtc, 2015-10-25 18:48:28]

Thanks for the contribution!

https://codereview.webrtc.org/1425583002/diff/1/BUILD.gn
File BUILD.gn (right):

https://codereview.webrtc.org/1425583002/diff/1/BUILD.gn#newcode13
BUILD.gn:13: "//webrtc/sound:rtc_sound",
Please put this into webrtc/BUILD.gn instead (the 'webrtc' source set).

[tfarina, 2015-10-25 21:01:44]

https://codereview.webrtc.org/1425583002/diff/1/BUILD.gn
File BUILD.gn (right):

https://codereview.webrtc.org/1425583002/diff/1/BUILD.gn#newcode13
BUILD.gn:13: "//webrtc/sound:rtc_sound",
On 2015/10/25 18:48:28, kjellander (webrtc) wrote:
> Please put this into webrtc/BUILD.gn instead (the 'webrtc' source set).

Done.

[kjellander_webrtc, 2015-10-26 04:19:56]

lgtm!

[tfarina, 2015-10-26 12:14:14]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-10-26 12:14:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1425583002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1425583002/20001

[commit-bot: I haz the power, 2015-10-26 12:15:31]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-26 12:15:42]

Patchset 2 (id:??) landed as
https://crrev.com/f054819e257a4f9cbb7fa82ba51dc2335f4359ec
Cr-Commit-Position: refs/heads/master@{#10405}