Add a 'gyp_only' GYP target for the GN migration.

Add a 'gyp_only' GYP target for the GN migration.

There are a few targets we build in a GYP build of Chromium that
we don't actually need and shouldn't be building. We will track
them in a 'gyp_only' target to indicate that they don't need to
be ported to GN.

TBR=brettw@chromium.org
BUG=461019
Committed: https://crrev.com/aa9e60e1e08b558a89d0883cdc807ea48f934492
Cr-Commit-Position: refs/heads/master@{#323170}

[brettw, 2015-03-31 18:11:10]

lgtm

[Dirk Pranke, 2015-03-31 19:29:26]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-03-31 19:30:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1041163004/20001

[commit-bot: I haz the power, 2015-03-31 19:35:51]

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

[commit-bot: I haz the power, 2015-03-31 19:35:52]

Try jobs failed on following builders:
  ios_dbg_simulator_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)

[Dirk Pranke, 2015-04-01 00:50:16]

The CQ bit was checked by dpranke@chromium.org

[Dirk Pranke, 2015-04-01 00:50:16]

The patchset sent to the CQ was uploaded after l-g-t-m from brettw@chromium.org
Link to the patchset: https://codereview.chromium.org/1041163004/#ps60001
(title: "merge to #323155")

[commit-bot: I haz the power, 2015-04-01 00:50:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1041163004/60001

[commit-bot: I haz the power, 2015-04-01 02:43:50]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-04-01 02:44:45]

Patchset 4 (id:??) landed as
https://crrev.com/aa9e60e1e08b558a89d0883cdc807ea48f934492
Cr-Commit-Position: refs/heads/master@{#323170}