Add dougarnett to components/data_reduction_proxy owners

(transitively components/previews owner as well)

Bug: 
Change-Id: I651c65eccb60bddc1124b601e6c46323b3fdb26a
Reviewed-on: https://chromium-review.googlesource.com/747803Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Doug Arnett <dougarnett@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512967}

components/data_reduction_proxy/OWNERS

 bengr@chromium.org
+dougarnett@chromium.org
 megjablon@chromium.org
 rajendrant@chromium.org
 ryansturm@chromium.org
@@ -11,4 +12,4 @@ per-file *_messages.cc=file://ipc/SECURITY_OWNERS
 per-file *_messages*.h=set noparent
 per-file *_messages*.h=file://ipc/SECURITY_OWNERS
 
-# COMPONENT: Internals>Network>DataProxy
+# COMPONENT: Internals>Network>DataProxy
\ No newline at end of file