gn_migration.gypi: Do not pull aura_unittests_run unless use_aura==1.

gn_migration.gypi: Do not pull aura_unittests_run unless use_aura==1.

This was a regression in 49f497d7d9c9c6c665abec0ddfbc0df07c3cd6c9 which caused
compilation failure when building everything on OSX.

TBR=dpranke@chromium.org
BUG=98637
NOTRY=true

Committed: https://crrev.com/e0f3d6391782f114ddd35babfff4b5f1fd9b61d6
Cr-Commit-Position: refs/heads/master@{#336802}

[sky, 2015-06-30 17:07:48]

sky@chromium.org changed reviewers:
+ sky@chromium.org

[sky, 2015-06-30 17:07:48]

LGTM

[M-A Ruel, 2015-06-30 17:08:42]

The CQ bit was checked by maruel@chromium.org

[commit-bot: I haz the power, 2015-06-30 17:09:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1220733009/1

[commit-bot: I haz the power, 2015-06-30 17:15:45]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-30 17:16:39]

Patchset 1 (id:??) landed as
https://crrev.com/e0f3d6391782f114ddd35babfff4b5f1fd9b61d6
Cr-Commit-Position: refs/heads/master@{#336802}