Migrate off base::ThreadPool()-as-a-trait in /content/browser/browser_main_loop_unittest.cc

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.

R=boliu@chromium.org

Bug: 1026641
Change-Id: I165a26f5d13f84d1f7ef03228e5faa34fe7c9a86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078660
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745484}

content/browser/browser_main_loop_unittest.cc

@@ -71,7 +71,7 @@ TEST_F(BrowserMainLoopTest, CreateThreadsInSingleProcess) {
   browser_main_loop.CreateThreads();
   EXPECT_GE(base::ThreadPoolInstance::Get()
                 ->GetMaxConcurrentNonBlockedTasksWithTraitsDeprecated(
-                    {base::ThreadPool(), base::TaskPriority::USER_VISIBLE}),
+                    {base::TaskPriority::USER_VISIBLE}),
             base::SysInfo::NumberOfProcessors() - 1);
   browser_main_loop.ShutdownThreadsAndCleanUp();
   BrowserTaskExecutor::ResetForTesting();