Fix extension command service to check if user has modified key before overwrite on update.

Worked for all other keys except for the media keys where CanAutoAssign would ALWAYS return true even if key was modified by user. Now no commands should be overwritten on update if they have been changed by a user.

BUG=421482
R=finnur@chromium.org

Review URL: https://codereview.chromium.org/667833004

Cr-Commit-Position: refs/heads/master@{#301309}

diff --git a/AUTHORS b/AUTHORS
index f2ab876..172a3ad 100644
--- a/AUTHORS
+++ b/AUTHORS

@@ -20,6 +20,7 @@
 Ajay Berwal <ajay.berwal@samsung.com>
 Ajith Kumar V <ajith.v@samsung.com>
 Aku Kotkavuo <a.kotkavuo@partner.samsung.com>
+Alex Gabriel <minilogo@gmail.com>
 Alex Gartrell <agartrell@cmu.edu>
 Alex Henrie <alexhenrie24@gmail.com>
 Alex Scheele <alexscheele@gmail.com>