Add sdefresne@ to ios/web/download/OWNERS.

Sylvain did codereviews for most of the CLs CL in this directory, which is
more than 5-10 reviews.

Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I120e618bed1f8ed1f6526c4ab204ad2341229191
Reviewed-on: https://chromium-review.googlesource.com/1246294
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594728}

ios/web/download/OWNERS

 eugenebut@chromium.org
+sdefresne@chromium.org
 
 # TEAM: ios-directory-owners@chromium.org
 # OS: iOS