Add OWNERS file for Chrome OS WindowSizer files.

Bug: none
Change-Id: I4bf6dc8b898a5c0af1516888e40e290e5bf91d95
Reviewed-on: https://chromium-review.googlesource.com/960982Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543001}

chrome/browser/ui/window_sizer/OWNERS

+per-file *ash*=estade@chromium.org
+per-file *ash*=jamescook@chromium.org