Fix nacl-only targets in gn_migration.gypi

Fix nacl-only targets in gn_migration.gypi

The 'nacl_loader_unittests' and 'remoting_key_tester'
targets require nacl and should only be built when
nacl is enabled.

TBR=dongseong.hwang@intel.com, tfarina@chromium.org
BUG=462945, 461019
Committed: https://crrev.com/ad333257fe8e7d9b632f728de5fe6db9feb9a64d
Cr-Commit-Position: refs/heads/master@{#318631}

[Dirk Pranke, 2015-03-01 20:22:13]

The CQ bit was checked by dpranke@chromium.org

[Dirk Pranke, 2015-03-01 20:22:46]

dpranke@chromium.org changed reviewers:
+ dongseong.hwang@intel.com, tfarina@chromium.org
- dshwang@chromium.org

[commit-bot: I haz the power, 2015-03-01 20:22:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/972563002/1

[tfarina, 2015-03-01 20:32:35]

lgtm

[commit-bot: I haz the power, 2015-03-01 21:17:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-01 21:18:17]

Patchset 1 (id:??) landed as
https://crrev.com/ad333257fe8e7d9b632f728de5fe6db9feb9a64d
Cr-Commit-Position: refs/heads/master@{#318631}