Add DisplayCompositorTest.

services/ui/surfaces/BUILD.gn

Index: services/ui/surfaces/BUILD.gn
diff --git a/services/ui/surfaces/BUILD.gn b/services/ui/surfaces/BUILD.gn
index aedccb55a2b64a40b1d5521a6fb879a5dad12a4f..08ebbac5c876cd491dd0978743b9e96fe8b725bb 100644
--- a/services/ui/surfaces/BUILD.gn
+++ b/services/ui/surfaces/BUILD.gn
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
+import("//testing/test.gni")
 
 source_set("surfaces") {
   sources = [
@@ -45,3 +46,18 @@ source_set("surfaces") {
     deps += [ "//gpu/command_buffer/common" ]
   }
 }
+
+test("mus_surfaces_unittests") {
+  sources = [
+    "display_compositor_unittest.cc",
+  ]
+
+  deps = [
+    ":surfaces",
+    "//base",
+    "//base/test:test_support",
+    "//cc:cc",
+    "//services/service_manager/public/cpp/test:run_all_service_tests",
+    "//testing/gtest",
+  ]
+}