Move image_unittest_util* into gfx_test_support target.

Move image_unittest_util* into gfx_test_support target.

That way we can stop building it within unit_tests and gfx_unittests and
share with them through gfx_test_support target.

This way is much more standard in chromium code.

BUG=354137
TEST=unit_tests, gfx_unittests
R=sky@chromium.org

Committed: https://crrev.com/db2b7895e5739487fda1db0d99b113c364631773
Cr-Commit-Position: refs/heads/master@{#296004}

[sky, 2014-09-22 16:21:25]

LGTM

[tfarina, 2014-09-22 17:07:57]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2014-09-22 17:09:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/590873002/1

[commit-bot: I haz the power, 2014-09-22 17:15:37]

Committed patchset #1 (id:1) as 06fb49ac0ee756b60a6ee13e55f16266d62834be

[commit-bot: I haz the power, 2014-09-22 17:16:16]

Patchset 1 (id:??) landed as
https://crrev.com/db2b7895e5739487fda1db0d99b113c364631773
Cr-Commit-Position: refs/heads/master@{#296004}