project.config: Make +2's sticky (again).

The changes:
https://dawn-review.googlesource.com/c/dawn/+/69404
https://dawn-review.googlesource.com/c/tint/+/69405

Changed the behavior of +2s, and now the "Code-Review" label is
dropped with trivial changes. The old behavior for both the 'dawn'
and 'tint' repos was to keep the +2. This CL restores that behavior.

Bug: b/203790428
Change-Id: I46a3393c58244763b09c612fdcd6d35ab9c837f1