AW: add OWNERS for aw/nonembedded subfolder

This adds an OWNERS file for the aw/nonembedded folder. I'm nominating
myself and torne@ for initial owners since we've written/reviewed a
number of CLs in the folder recently, but this still inherits the
top-level AW owners.

Test: N/A
Change-Id: Ic4591f59e1a4e81088ea0de69136e1dcc39d4b10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1976882Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726578}

android_webview/nonembedded/OWNERS

+ntfschr@chromium.org
+torne@chromium.org