[val] Use a helper to build the task queue

The task queue gets built in reverse order, so it's hard to reason about
the written code because it's backwards. This CL adds a helper which
takes a set of lambdas and does the work to add them to the task queue
in the necessary order.

This allows the code to read in the correct order while creating the
task queue in the reverse order.

Bug: 459529440
Change-Id: Ie31217df6d3164cd51593054cc01a0503e300ab7
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/336655
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>