Fix components/undo/BUILD.gn

Fix components/undo/BUILD.gn

Change reference to sibling target from absolute to relative to follow
the best practices.

BUG=476917
Committed: https://crrev.com/8b64a19080bd938333dc69fba0ea1c53d24a162f
Cr-Commit-Position: refs/heads/master@{#325833}

[sdefresne, 2015-04-20 08:42:57]

sdefresne@chromium.org changed reviewers:
+ droger@chromium.org, tfarina@chromium.org

[droger, 2015-04-20 11:40:00]

This is indeed what I meant in my earlier comment.

LGTM

[sdefresne, 2015-04-20 12:11:55]

The CQ bit was checked by sdefresne@chromium.org

[commit-bot: I haz the power, 2015-04-20 12:12:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1097773005/1

[commit-bot: I haz the power, 2015-04-20 12:15:07]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-20 12:15:57]

Patchset 1 (id:??) landed as
https://crrev.com/8b64a19080bd938333dc69fba0ea1c53d24a162f
Cr-Commit-Position: refs/heads/master@{#325833}

[tfarina, 2015-04-20 12:41:59]

lgtm

https://codereview.chromium.org/1097773005/diff/1/components/undo/BUILD.gn
File components/undo/BUILD.gn (right):

https://codereview.chromium.org/1097773005/diff/1/components/undo/BUILD.gn#ne...
components/undo/BUILD.gn:37: ":undo",
I know sort would but this last, but in general we put it at the top before the
other deps.