Add mgiuca as OWNER of chrome/browser/external_protocol

Change-Id: I5eb58a1862611fb9a23396c1852de90c23153a78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210280
Auto-Submit: Matt Giuca <mgiuca@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770902}

chrome/browser/external_protocol/OWNERS

 benwells@chromium.org
 dominickn@chromium.org
 meacer@chromium.org
+mgiuca@chromium.org
 # COMPONENT: UI>Browser>WebAppInstalls