Add Hexcles--robertma to handle mapping

First CL! :)

Bug: 
Change-Id: I171a77c0d0d99316cc9d2ce3ac7aa4fdcc83fa9d
Reviewed-on: https://chromium-review.googlesource.com/575703
Reviewed-by: Rick Byers <[email protected]>
Commit-Queue: Robert Ma <[email protected]>
Cr-Commit-Position: refs/heads/master@{#487929}
diff --git a/docs/user_handle_mapping.md b/docs/user_handle_mapping.md
index 77916dec..2b2c7cb 100644
--- a/docs/user_handle_mapping.md
+++ b/docs/user_handle_mapping.md
@@ -110,3 +110,4 @@
 | wjmaclean         | seumas, wjmaclean | wjmaclean, wjm, seumas |
 | yoz               | yaws            | yoz             |
 | zmo               | zhenyao         | zmo             |
+| robertma          | Hexcles         | robertma        |