bookmarks: Add mastiz@ to OWNERS file

Change-Id: Ic9e7222d268da4c7eb1b402f66550f290092371c
Bug: None
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2108505
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751501}

components/bookmarks/OWNERS

 sky@chromium.org
+mastiz@chromium.org
 
 # COMPONENT: UI>Browser>Bookmarks