Modify //tools/mb/OWNERS file

Change-Id: I89fa7d5596688f676d60d6f58b82383ee60cefab
Reviewed-on: https://chromium-review.googlesource.com/1197729
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587900}

tools/mb/OWNERS

+bpastene@chromium.org
 dpranke@chromium.org
 jbudorick@chromium.org
+kbr@chromium.org
+martiniss@chromium.org
 scottmg@chromium.org
-tansell@chromium.org
 
 # TEAM: infra-dev@chromium.org
 # COMPONENT: Build