Add alexst to chromecast/OWNERS

Bug: None
Change-Id: I35be980d4b325f804ae4cb49b471bbd5ef55d6e9
Reviewed-on: https://chromium-review.googlesource.com/1028134Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Reviewed-by: Stephen Lanham <slan@chromium.org>
Commit-Queue: Alex Sakhartchouk <alexst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553649}

chromecast/OWNERS

@@ -3,6 +3,7 @@ servolk@chromium.org
 slan@chromium.org
 
 # For major changes, please use one of the reviewers above!
+alexst@chromium.org
 spang@chromium.org
 
 # COMPONENT: Chromecast