Nominate myself for content/renderer/OWNERS

It makes sense to have someone from the rendering team in OWNERS.
I'm nominating myself because my code review load is not too heavy,
I'm reasonably familiar with the code, and I anticipate working more
in this area going forward.

R=dcheng,haraken,eae,chrishtr

Change-Id: Id3141c2aa100a26afaef1dd07980fbb63dea7945
Reviewed-on: https://chromium-review.googlesource.com/c/1490632Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635832}

content/renderer/OWNERS

 dcheng@chromium.org
 haraken@chromium.org
+szager@chromium.org
 
 # These are for the common case of adding or renaming files. If you're doing
 # structural changes, please get a review from a reviewer in this file.