GN: Refactor modules_unittests to eliminate package boundary violations.

webrtc/modules/utility/BUILD.gn

Index: webrtc/modules/utility/BUILD.gn
diff --git a/webrtc/modules/utility/BUILD.gn b/webrtc/modules/utility/BUILD.gn
index 48cbf793ed5ebf5b57c53f2c4039c717c8d13d7c..5b6a232adc18d5836b5b39788baa2bfbdbf1b3b4 100644
--- a/webrtc/modules/utility/BUILD.gn
+++ b/webrtc/modules/utility/BUILD.gn
@@ -42,3 +42,18 @@ rtc_static_library("utility") {
     "../media_file",
   ]
 }
+
+if (rtc_include_tests) {
+  rtc_source_set("utility_unittests") {
+    testonly = true
+    sources = [
+      "source/process_thread_impl_unittest.cc",
+    ]
+    deps = [
+      ":utility",
+      "../../base:rtc_task_queue",
+      "../../test:test_support",
+      "//testing/gmock",
+    ]
+  }
+}