[serial] Add OWNERS file for code in chrome/browser/ui/serial

Assigns ownership of this directory to the master list of Serial API
owners in //content/browser/serial.

Change-Id: I9b9cc4410658d66f2a9293b2e732f8cd264a4f7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2144829
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758066}

chrome/browser/ui/serial/OWNERS

+file://content/browser/serial/OWNERS
+
+# COMPONENT: Blink>Serial