[PartitionAlloc] Add bartekn@ to OWNERS

He might know this and that about PartitionAlloc.

$ git log base/allocator/partition_allocator/ | grep Author | grep bartekn | wc -l
43
$ git log base/allocator/partition_allocator/ | grep Reviewed-by | grep bartekn | wc -l
58

Change-Id: I8f6ccec8d82e1950db26b4d7ad7c8f554d63fd1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2534356Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826804}

base/allocator/partition_allocator/OWNERS

 ajwong@chromium.org
+bartekn@chromium.org
 haraken@chromium.org
 lizeb@chromium.org
 palmer@chromium.org