[ThreadPool]: TaskSource::TakeTask to return Task instead of Optional<Task>

Change-Id: Ic9ec415c89d5637ccc992abeeacb4b72c847411a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906629
Reviewed-by: Fran├žois Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Fran├žois Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714077}
10 files changed