Update Partition Alloc OWNERS file.

New onwers, more specific label, additional team.

Bug: None
Change-Id: Id4a3ea39b4f57b68b2d115ad38730c7e153a65cb
Reviewed-on: https://chromium-review.googlesource.com/843238Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527074}

base/allocator/partition_allocator/OWNERS

+ajwong@chromium.org
 haraken@chromium.org
 palmer@chromium.org
+tsepez@chromium.org
 
 # TEAM: platform-architecture-dev@chromium.org
-# COMPONENT: Blink>MemoryAllocator
+#       Also: security-dev@chromium.org
+# COMPONENT: Blink>MemoryAllocator>Partition