Migrate off base::ThreadPool()-as-a-trait in /chrome/browser/resource_coordinator

This CL is a logical no-op.

It manually covers the use cases the script
(https://crbug.com/1026641#c22) didn't cover in
https://chromium-review.googlesource.com/c/chromium/src/+/2026350.

A few of the affected files were also migrated to
content::GetUIThreadTaskRunner() whenever surrounding code consistency
was at stake (but the bulk of that migration is staged to happen after
this one so this wasn't a priority in this batch).

Most files intentionally kept base/task/post_task.h as an include. It is
still required because of a cyclical dependency during the migration
preventing base/task/thread_pool.h from implicitly providing
base/task/task_traits.h to its users (callers that explicitly store
TaskTraits and include task_traits.h don't have this problem). The
post_task.h include will be cleaned up in a later phase.

This CL was uploaded by git cl split.

TBR=zhenw@chromium.org

Bug: 1026641
Change-Id: Ie10e43eb49833e02e9dd9dd2b92a9ff1e51f22e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078780Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745942}

chrome/browser/resource_coordinator/leveldb_site_characteristics_database.cc

@@ -397,11 +397,10 @@ LevelDBSiteCharacteristicsDatabase::AsyncHelper::OpenOrCreateDatabaseImpl() {
 
 LevelDBSiteCharacteristicsDatabase::LevelDBSiteCharacteristicsDatabase(
     const base::FilePath& db_path)
-    : blocking_task_runner_(base::CreateSequencedTaskRunner(
+    : blocking_task_runner_(base::ThreadPool::CreateSequencedTaskRunner(
           // The |BLOCK_SHUTDOWN| trait is required to ensure that a clearing of
           // the database won't be skipped.
-          {base::ThreadPool(), base::MayBlock(),
-           base::TaskShutdownBehavior::BLOCK_SHUTDOWN})),
+          {base::MayBlock(), base::TaskShutdownBehavior::BLOCK_SHUTDOWN})),
       async_helper_(new AsyncHelper(db_path),
                     base::OnTaskRunnerDeleter(blocking_task_runner_)) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);