Allow any committer to upload new patchsets to other people's CLs

AFAIK all of the permissions checks work fine when this happens. I have
access to this permission via webgpu-dawn-admin and I use it and have
observed it working as expected (trust belongs to the latest uploader,
not the original author). Currently we have this restricted to just
admins, but Gerrit by default allows *all* users to do this:
https://gerrit-review.googlesource.com/Documentation/access-control.html#category_add_patch_set

This is useful for various things but mainly fixing trivial stuff in
external contributors' CLs before landing.

Bug: none
Change-Id: I2cdc65796128c4c2b65f6ba2c19f387136523707