Update OWNERS for prerender and prefetch

Change-Id: If8752e9b7a61d59537a528bd71d45462656ab9ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032423
Auto-Submit: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737302}

chrome/browser/prefetch/OWNERS

-gavinp@chromium.org
-pasko@chromium.org
+file://chrome/browser/prerender/OWNERS
+
 # COMPONENT: Internals>Preload

chrome/browser/prerender/OWNERS

-# New owners ramping up on prerender.
+robertogden@chromium.org
 ryansturm@chromium.org
 tbansal@chromium.org