So don't build it for all platforms and non-test targets.

BUG=778369
TEST=none

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I106f2a909b27028fbc88a2fdf9dd850f97d5eb73
Reviewed-on: https://chromium-review.googlesource.com/738638


Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513391}