Related spec pr: gpuweb/gpuweb#1811
Dawn issue: crbug.com/dawn/984

Author checklist for test code/plans:

• All outstanding work is tracked with "TODO" in a test/file description or .unimplemented() on a test.
• New helpers, if any, are documented using /** doc comments */ and can be found via helper_index.txt.
• (Optional, sometimes not possible.) Tests pass (or partially pass without unexpected issues) in an implementation. (Add any extra details above.)

Reviewer sign-off for test code/plans: (Note: feel free to pull in other reviewers at any time for any reason.)

• The test path is reasonable, the description "describes the test, succinctly, but in enough detail that a reader can read only the test plans in a file or directory and evaluate the completeness of the test coverage."
• Tests appear to cover this area completely, except for outstanding TODOs. Validation tests use control cases.
  (This is critical for coverage. Assume anything without a TODO will be forgotten about forever.)
• Existing (or new) test helpers are used where they would reduce complexity.
• TypeScript code is readable and understandable (is unobtrusive, has reasonable type-safety/verbosity/dynamicity).