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ThreadPool: Extract ThreadPoolImpl tests that don't need to run for all traits/execution modes.

ThreadPoolImpl tests currently run for each combination of:
- thread pool type
- traits
- execution mode

In preparation for a CL that adds new traits, this CL extracts tests
that don't query the traits and execution mode to a separate test
class that only runs for each thread pool type.

Change-Id: Ic8406101eeb10288dd5e797747ec2be3fc8f654a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626872Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663088}