Update build files to use webrtc_overrides in Chromium instead of overrides.

Update build files to use webrtc_overrides in Chromium instead of overrides.

This re-lands https://codereview.webrtc.org/1354933002/

This is a part of moving the overrides to Chromium. See bug comment #65 for all steps.

Depends on https://codereview.chromium.org/1345873004/

BUG=chromium:468375
Committed: https://crrev.com/ee2bf41a79b9bf44aa1658d2d491577ae440334e
Cr-Commit-Position: refs/heads/master@{#10115}

[Henrik Grunell WebRTC, 2015-09-21 10:55:06]

henrikg@webrtc.org changed reviewers:
+ kjellander@chromium.org

[Henrik Grunell WebRTC, 2015-09-21 11:03:19]

henrikg@webrtc.org changed reviewers:
+ kjellander@webrtc.org
- kjellander@chromium.org

[kjellander_webrtc, 2015-09-21 12:42:53]

lgtm

[Henrik Grunell WebRTC, 2015-09-30 10:00:33]

The CQ bit was checked by henrikg@webrtc.org

[Henrik Grunell WebRTC, 2015-09-30 10:00:33]

The patchset sent to the CQ was uploaded after l-g-t-m from
kjellander@webrtc.org
Link to the patchset: https://codereview.webrtc.org/1345313004/#ps20001 (title:
"Rebase.")

[commit-bot: I haz the power, 2015-09-30 10:00:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1345313004/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1345313004/20001

[commit-bot: I haz the power, 2015-09-30 10:48:58]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-30 10:49:18]

Patchset 2 (id:??) landed as
https://crrev.com/ee2bf41a79b9bf44aa1658d2d491577ae440334e
Cr-Commit-Position: refs/heads/master@{#10115}