Tentative proposal:

Change return type of base::CreateFooTaskRunnerWithTraits() from FooTaskRunner to AdjustableFooTaskRunner.

An AdjustableFooTaskRunner is-a FooTaskRunner and can be passed as such to components that shouldn't be able to toggle the sequence's traits.

By default though the owner of the original task runner may toggle its TaskTraits at run-time if it holds on to the AdjustableFooTaskRunner interface.

Use cases:
 * Windows module enumeration : BACKGROUND set of tasks that may become USER_BLOCKING if the user opens a special chrome:// URL

 * Read on startup, forever after periodically flush to disk : read is USER_BLOCKING/CONTINUE_ON_SHUTDOWN, write is BACKGROUND/BLOCK_ON_SHUTDOWN.
  Currently the whole sequence must remain USER_BLOCKING priority for all writes after startup, this is unnecessary and can cause random flushes to race with the cache on page load.
  Not being to use CONTINUE_ON_SHUTDOWN on startup has resulted in shutdown hangs when shutting down before startup is complete (e.g. issue 477409).