[base] Rename TaskScheduler to ThreadPool

Reason: with the advent of other scheduling primitives in //base
(i.e. SequenceManager), TaskScheduler was no longer the only component
responsible for scheduling tasks. We will from now on refer to the
whole of //base/task as the "task scheduling infrastructure".

There are other types named "TaskScheduler" outside of base:: so
s/TaskScheduler/ThreadPool/ across the codebase wasn't possible.

Instead, this CL did:
 1) base/task/task_scheduler => base/task/thread_pool
    (catches all files with includes)
 1.1) Careful manual search to add files without includes
      (e.g. missing IWYU, docs, etc.)
 2) TaskScheduler => ThreadPool in all files affected by (1)
 3) task_scheduler => thread_pool in all files affected by (1)
 4) "task scheduler" => "thread pool"  in all files affected by (1)
 4) Move task_scheduler_util like headers in
    //content //components and //ios

Also:
 * Renamed UMA metrics from TaskScheduler.* to ThreadPool.*
   and dropped "Pool" from worker pool name suffixes.
 * Renamed TaskScheduler*Worker thread names to ThreadPool*Worker
 * In base/android: NativeTaskScheduler => NativeScheduler as it
   was referring to the whole of base/task.
   TaskSchedulerTest.java => NativePostTaskTest.java (former DNE)
 * Intentionally ignoring IWYU violations in this already too large
   CL.

In follow-up:
 * Rename other types as well:
     SchedulerWorker => WorkerThread
     SchedulerWorkerPool* => WorkerThreadGroup*

Bug: 951388
Change-Id: I5bc2688b593c7682ef7e56d6b228539970ba107e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1561552
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@google.com>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650997}

diff --git a/content/browser/after_startup_task_utils.h b/content/browser/after_startup_task_utils.h
index 7debbad..6df9748 100644
--- a/content/browser/after_startup_task_utils.h
+++ b/content/browser/after_startup_task_utils.h

@@ -13,7 +13,7 @@
 // ContentBrowserClient::SetBrowserStartupIsCompleteForTesting() as that can
 // only be called from the content implementation and including content/test in
 // the content implementation is a whole other can of worms. TODO(gab): Clean
-// this up when AfterStartupTasks go away in favor of TaskScheduler.
+// this up when AfterStartupTasks go away in favor of ThreadPool.
 void CONTENT_EXPORT SetBrowserStartupIsCompleteForTesting();
 
 }  // namespace content