Prerender owners update (+ryansturm, -bengr)

This change updates the c/b/prerender/OWNERS file to add ryansturm and
remove bengr.

Change-Id: I475acaca1497d0698d62325da75d5d6d3ddae387
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1709353Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678829}

chrome/browser/prerender/OWNERS

 # New owners ramping up on prerender.
+ryansturm@chromium.org
 tbansal@chromium.org
-bengr@chromium.org
 
 # Deprecated reviewers.
 lizeb@chromium.org