Add components/keyed_service to GN build.

BUG=
[email protected]

Review URL: https://codereview.chromium.org/336443004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278514 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/components/keyed_service.gypi b/components/keyed_service.gypi
index b03a204..2f0ceef 100644
--- a/components/keyed_service.gypi
+++ b/components/keyed_service.gypi
@@ -5,6 +5,7 @@
 {
   'targets': [
     {
+      # GN version: //components/keyed_service/core:core
       'target_name': 'keyed_service_core',
       'type': '<(component)',
       'defines': [
@@ -32,6 +33,7 @@
     ['OS != "ios"', {
       'targets': [
         {
+          # GN version: //components/keyed_service/content:content
           'target_name': 'keyed_service_content',
           'type': '<(component)',
           'defines': [