Add robertogden as OWNERS in isolated prerender directory

This is redundant, but will help direct changes to the right reviewer.

Change-Id: Ic4e92f3364d503bbe66864011efc249065592df8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488704Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819123}

chrome/browser/prerender/isolated/OWNERS

+robertogden@chromium.org