GN build rules for all unit test targets in mojo/

mojo/mojo_services.gypi

Index: mojo/mojo_services.gypi
diff --git a/mojo/mojo_services.gypi b/mojo/mojo_services.gypi
index 20516bad355cf72ddf1b57fefcec3ca80bb0fa3c..16204e7fb8c167cae7de3e57c9db352dfcb96423 100644
--- a/mojo/mojo_services.gypi
+++ b/mojo/mojo_services.gypi
@@ -529,6 +529,7 @@
       ],
     },
     {
+      # GN version: //mojo/services/public/cpp/view_manager/tests:mojo_view_manager_lib_unittests
       'target_name': 'mojo_view_manager_lib_unittests',
       'type': 'executable',
       'dependencies': [
@@ -726,6 +727,7 @@
           ],
         },
         {
+          # GN version: //mojo/services/public/cpp/view_manager/lib:run_unittests
           'target_name': 'mojo_view_manager_run_unittests',
           'type': 'static_library',
           'dependencies': [
@@ -753,6 +755,7 @@
           ],
         },
         {
+          # GN version: //mojo/services/view_manager:mojo_view_manager_unittests
           'target_name': 'mojo_view_manager_unittests',
           'type': 'executable',
           'dependencies': [
@@ -789,6 +792,7 @@
           'includes': [ 'build/package_app.gypi' ],
         },
         {
+          # GN version: //mojo/services/window_manager:lib
           'target_name': 'mojo_core_window_manager_lib',
           'type': 'static_library',
           'dependencies': [
@@ -812,6 +816,7 @@
           ],
         },
         {
+          # GN version: //mojo/services/window_manager
           'target_name': 'mojo_core_window_manager',
           'type': 'loadable_module',
           'dependencies': [
@@ -823,6 +828,7 @@
           ],
         },
         {
+          # GN version: //mojo/services/window_manager:mojo_core_window_manager_unittests
           'target_name': 'mojo_core_window_manager_unittests',
           'type': 'executable',
           'dependencies': [