[threadpool] Add switch to log BEST_EFFORT tasks.

When the --log-best-effort-tasks switch is specified, information about
all tasks posted with BEST_EFFORT priority is logged. This is useful to
diagnose issues with preventing BEST_EFFORT tasks from running during
startup (and eventually during page load).

Bug: 887407
Change-Id: I366a78485c1588741519c7254f8e1ab57d1df140
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1660635
Reviewed-by: Avi Drissman <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Gabriel Charette <[email protected]>
Commit-Queue: François Doray <[email protected]>
Cr-Commit-Position: refs/heads/master@{#670886}
16 files changed