[OWNERS] revive agable

Change-Id: I67a063d59ae3a579e77a1a6817a66f9d5f1ea53d
Reviewed-on: https://chromium-review.googlesource.com/c/1480752Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>

OWNERS

 set noparent
 
-# unavail for rest of 2018
-# agable@chromium.org
-
+agable@chromium.org
 dpranke@chromium.org
 ehmaldonado@chromium.org
 hinoka@chromium.org