Drop `IntegrityMetadataPair`.

This CL turns `IntegrityMetadata` into a simple struct, and uses it
directly in `IntegrityMetadataSet` rather than converting it into a
`std::pair` along the way. This CL shouldn't cause any change in
behavior, it just simplifies a weird bit of infrastructure.

Change-Id: I3696aac46efbb04c4b1c01f9c1f149b7eb580f8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6400757
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Antonio Sartori <antoniosartori@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1439264}