Mac Overlays: Clear root layer's contents when it is not specified

Mac Overlays: Clear root layer's contents when it is not specified

If the root layer is not included in a frame, this is because that
frame is being drawn entirely by overlays. Set the root frame's contents
to nil, and delete references to it.

BUG=533687
Committed: https://crrev.com/b42c09e7f49efa478d05866ae5c58e94d8a630f3
Cr-Commit-Position: refs/heads/master@{#353140}

[ccameron, 2015-10-08 20:23:34]

ccameron@chromium.org changed reviewers:
+ andresantoso@chromium.org

[ccameron, 2015-10-08 20:23:34]

Small change. Ptal.

[Andre, 2015-10-08 20:26:09]

LGTM

[ccameron, 2015-10-08 20:45:50]

Thanks!

[ccameron, 2015-10-08 20:45:53]

The CQ bit was checked by ccameron@chromium.org

[commit-bot: I haz the power, 2015-10-08 20:47:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1393393002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1393393002/1

[commit-bot: I haz the power, 2015-10-08 21:19:22]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-08 21:20:21]

Patchset 1 (id:??) landed as
https://crrev.com/b42c09e7f49efa478d05866ae5c58e94d8a630f3
Cr-Commit-Position: refs/heads/master@{#353140}