Add infra owners to //third_party/blink/tools/blinkpy/common/config/builders.json.

This file needs to be updated when we add or remove builders, so
it makes sense to have infra troopers be able to make and approve
changes for it, along with the normal owners of the code.

Change-Id: Id3351654d75b23019b59062c124e7f2d2fa29e62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2342264
Commit-Queue: Dirk Pranke <dpranke@google.com>
Commit-Queue: Erik Staab <estaab@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795765}

third_party/blink/tools/blinkpy/common/config/OWNERS

+# builders.json needs to be changed whenever we add or remove builders
+# from the waterfalls.
+#
+# So, it makes sense to let infra troopers be owners of it as well, rather
+# than having to route their changes through the .../tools OWNERS.
+
+per-file builders.json=file://infra/OWNERS