content: Port layout_test_helper target to GN build.

content: Port layout_test_helper target to GN build.

This is necessary because it is one of the dependencies of the
'blink_tests' group that should be built.

BUG=483171
R=avi@chromium.org,dpranke@chromium.org

Committed: https://crrev.com/10ae51f628f7447699baf16db52c5d74f9a96077
Cr-Commit-Position: refs/heads/master@{#329580}

[Avi (use Gerrit), 2015-05-12 15:02:34]

lgtm

[tfarina, 2015-05-12 20:11:32]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-05-12 20:11:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1131773004/1

[commit-bot: I haz the power, 2015-05-12 20:19:29]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-05-12 20:19:30]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[tfarina, 2015-05-12 20:30:23]

Dirk, I need your approval for src/BUILD.gn

[Dirk Pranke, 2015-05-13 00:41:04]

lgtm

[tfarina, 2015-05-13 03:31:51]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-05-13 03:32:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1131773004/1

[commit-bot: I haz the power, 2015-05-13 03:36:07]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-13 03:36:48]

Patchset 1 (id:??) landed as
https://crrev.com/10ae51f628f7447699baf16db52c5d74f9a96077
Cr-Commit-Position: refs/heads/master@{#329580}