Fix chromiumos breakage for gn_all.

Fix chromiumos breakage for gn_all.

The gn_all target in all.gyp is attempting to build ':image_diff#host',
which I think may not work correctly in a chromeos==1 build. I think
the '...#host' syntax should probably only be used on the android port
(looks like that's what content_shell.gypi does). Though, given that
we're building content_shell, we shouldn't need the image_diff
dependency at all, so I'm just deleting it outright.

TBR=abodenha@chromium.org, brettw@chromium.org, hshi@chromium.org
NOTRY=true
NOTREECHECKS=true
BUG=461602
Committed: https://crrev.com/88ba00883a3d5f9b8b9fbc498d444d4f61b7560f
Cr-Commit-Position: refs/heads/master@{#317953}

[Dirk Pranke, 2015-02-25 01:15:21]

Patchset #1 (id:1) has been deleted

[Dirk Pranke, 2015-02-25 01:15:34]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-02-25 01:16:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/951553004/20001

[Dirk Pranke, 2015-02-25 02:10:40]

The CQ bit was unchecked by dpranke@chromium.org

[Dirk Pranke, 2015-02-25 02:11:59]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-02-25 02:12:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/951553004/40001

[Dirk Pranke, 2015-02-25 02:13:29]

dpranke@chromium.org changed reviewers:
+ hshi@chromium.org

[Dirk Pranke, 2015-02-25 02:13:43]

The CQ bit was unchecked by dpranke@chromium.org

[Dirk Pranke, 2015-02-25 02:13:45]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-02-25 02:14:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/951553004/40001

[commit-bot: I haz the power, 2015-02-25 02:15:49]

Committed patchset #2 (id:40001)

[commit-bot: I haz the power, 2015-02-25 02:16:17]

Patchset 2 (id:??) landed as
https://crrev.com/88ba00883a3d5f9b8b9fbc498d444d4f61b7560f
Cr-Commit-Position: refs/heads/master@{#317953}