[owners] Add ishell to base/OWNERS

Change-Id: Ic7c829233668c0592c027d351f02890091d7acbd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2300479Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68938}

src/base/OWNERS

 clemensb@chromium.org
+ishell@chromium.org
 mlippautz@chromium.org
 
 # COMPONENT: Blink>JavaScript