Refactor BUILD.gn from dwa to private_metrics

This CL moves dwa/BUILD.gn to private_metrics/BUILD.gn and fixes the
references in the Chromium code base.

Bug: 433729993
Change-Id: Icb18514dc83b9a1cd6099caa96dd9f53e1481f2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6966657
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Commit-Queue: Jay Zhou <zhouzj@google.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1517935}
NOKEYCHECK=True
GitOrigin-RevId: 8fb2e6804aff8dc8b11c5b9153708c52a3d8d43d