ThreadPool: Reduce the number of configurable options.

With this CL:

//base:
- The maximum number of BEST_EFFORT tasks that can run concurrently
  when there are no ScopedBlockingCalls is hard-coded to 2.
- The reclaim time has a default value of 30 seconds in
  ThreadPool::InitParams. It should not be overridden outside of tests.
- The WorkerThreadBackwardCompatibility enum is merged with the
  WorkerEnvironment enum. This makes sense because choosing a
  non-default value in both enums wasn't supported.
- A HAS_NATIVE_THREAD_POOL() macro is added to avoid duplication of the
  list of platforms that support native pools.

//content:
- The maximum number of threads in the foreground pool is hard-coded
  in //content. It is no longer possible to change it via a field
  trial. If we decide to experiment again with this, we should read
  field trial values directly from //base instead of using
  Content(Browser|Renderer)Client.

Change-Id: I667b5fb946b62756a0d78ca297bcfdc6b3d8fb0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600838
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660360}
61 files changed