Enable building WebRTC with GN.

Enable building WebRTC with GN.

This CL depends on the following CL to be landed first: 
https://webrtc-codereview.appspot.com/15989004
with the addition that that WebRTC revision of it needs to be
be rolled into Chromium DEPS first.

BUG=webrtc:3441
TEST=See https://webrtc-codereview.appspot.com/15989004 for details.

Committed: https://crrev.com/91966d30b3d11da8437d8499440e5bb7555b4530
Cr-Commit-Position: refs/heads/master@{#291688}

[brettw, 2014-06-11 17:26:06]

lgtm

[kjellander_chromium, 2014-07-02 07:42:40]

I'm waiting for https://code.google.com/p/webrtc/source/detail?r=6560 to be
rolled in to webrtc_revision of Chromium's DEPS before trying this again. I
imagine it will solve the dbus-glib errors seen on the trybots.

[kjellander_chromium, 2014-07-04 11:03:42]

On 2014/07/02 07:42:40, kjellander (OOO til Aug 4) wrote:
> I'm waiting for https://code.google.com/p/webrtc/source/detail?r=6560 to be
> rolled in to webrtc_revision of Chromium's DEPS before trying this again. I
> imagine it will solve the dbus-glib errors seen on the trybots.

Another CL in WebRTC is needed to get this to compile:
https://webrtc-codereview.appspot.com/15989004 
Building the 'webrtc' target in Chromium is not enough since there are other
targets that get different include paths. Building the default target (all) is
necessary.

[kjellander_chromium, 2014-08-25 15:13:35]

The CQ bit was checked by kjellander@chromium.org

[commit-bot: I haz the power, 2014-08-25 15:14:16]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/kjellander@chromium.org/321313006/80001

[commit-bot: I haz the power, 2014-08-25 16:08:11]

Committed patchset #4 (80001) as 192f92ed2d454213cafd61d2dc2fb45bdf6e190a

[commit-bot: I haz the power, 2014-09-10 02:34:44]

Patchset 4 (id:??) landed as
https://crrev.com/91966d30b3d11da8437d8499440e5bb7555b4530
Cr-Commit-Position: refs/heads/master@{#291688}