WebUI generator GN build file fixed.

WebUI generator GN build file fixed.

In case when GN output dir is inside src/, GN uses pathes relative to src/
directory that start with // (//out/Release/gen). But python interprets such
pathes as relative to /.

BUG=459230
Committed: https://crrev.com/240be55564f8d82c1e8281e052f15f6258185068
Cr-Commit-Position: refs/heads/master@{#319058}

[dzhioev (left Google), 2015-03-03 19:39:29]

dzhioev@chromium.org changed reviewers:
+ nkostylev@chromium.org

[Nikita (slow), 2015-03-04 12:16:14]

lgtm

[dzhioev (left Google), 2015-03-04 12:51:52]

The CQ bit was checked by dzhioev@chromium.org

[commit-bot: I haz the power, 2015-03-04 12:52:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/973873003/1

[commit-bot: I haz the power, 2015-03-04 13:16:01]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-04 13:16:33]

Patchset 1 (id:??) landed as
https://crrev.com/240be55564f8d82c1e8281e052f15f6258185068
Cr-Commit-Position: refs/heads/master@{#319058}