Fix GN build for junit tests

Fix GN build for junit tests

A dex path was required for all build types while for unit tests
(java_library) we don't provide one.

BUG=480445
Committed: https://crrev.com/d88c889bc2b6632810d7eb6f73ae8b4cf851168f
Cr-Commit-Position: refs/heads/master@{#326665}

[dgn, 2015-04-23 16:47:42]

dgn@chromium.org changed reviewers:
+ cjhopman@chromium.org

[dgn, 2015-04-23 16:47:43]

PTAL

[cjhopman, 2015-04-23 17:51:01]

lgtm

[cjhopman, 2015-04-23 20:52:19]

The CQ bit was checked by cjhopman@chromium.org

[commit-bot: I haz the power, 2015-04-23 20:52:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1083413010/1

[commit-bot: I haz the power, 2015-04-23 22:05:51]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-23 22:06:44]

Patchset 1 (id:??) landed as
https://crrev.com/d88c889bc2b6632810d7eb6f73ae8b4cf851168f
Cr-Commit-Position: refs/heads/master@{#326665}