Chromium

Generic bug for tracking in-flight work to use base::span more in //base.

Plan for progress here:
- Rewrite base APIs and their users and base implementations to use span.
- At some point start to verify that some parts of //base can be built with -Wunsafe-buffer-usage
  - To do that we can pull some .cc files out of base into a source_set and apply the warning to them.
  - This will show how to prevent backsliding, and we can do this until all of //base cc files pass, and apply the warning to base but not to things that depend on it yet (in GN configs but not all_dependent_configs)