Add svaldez to third_party/boringssl/OWNERS.

This is long long *long* overdue.

Bug: none
Change-Id: I9e72f5229ebfb3ea7676bd5e3b37d74b102fa6ee
Reviewed-on: https://chromium-review.googlesource.com/1052307Reviewed-by: Steven Valdez <svaldez@chromium.org>
Commit-Queue: Steven Valdez <svaldez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557210}

third_party/boringssl/OWNERS

 agl@chromium.org
 davidben@chromium.org
 rsleevi@chromium.org
+svaldez@chromium.org
 
 # COMPONENT: Internals>Network>SSL