window-service: Adds gpu_hosts tests back to services_unittests

Removing these was an oversight.

BUG=779614
TEST=test only change

Change-Id: I726deff7a1b25319f0b3f8740ddde88daa0262cd
Reviewed-on: https://chromium-review.googlesource.com/1190843
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Mohsen Izadi <mohsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586407}

services/ws/BUILD.gn

@@ -200,6 +200,7 @@ source_set("tests") {
     "//services/service_manager/public/cpp/test:test_support",
     "//services/service_manager/public/mojom",
     "//services/ws/common:task_runner_test_base",
+    "//services/ws/gpu_host:tests",
     "//services/ws/public/cpp",
     "//services/ws/public/mojom",
     "//testing/gtest",