Add schenney@ as libxml/libxslt OWNER

Also, just reuse the OWNERS file for libxml in libxslt.

Change-Id: I3cc3f0bc0a207090f86b72a8060565091d7f684a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2001109Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731761}

third_party/libxml/OWNERS

 # There's no real owners here. If you're familiar with the code please send
 # a CL to add yourself here.
 dcheng@chromium.org
+schenney@chromium.org
 palmer@chromium.org
 
 # COMPONENT: Blink>XML

third_party/libxslt/OWNERS

-dcheng@chromium.org
-palmer@chromium.org
+file://third_party/libxml/OWNERS
 # COMPONENT: Blink>XML