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

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=asvitkine@chromium.org

Bug: 1026641
Change-Id: I4ec15c2ebfc60e84bc8d8e2d7bdf6dc9f0f783b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079079Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745502}

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc

@@ -18,6 +18,7 @@
 #include "base/system/sys_info.h"
 #include "base/task/post_task.h"
 #include "base/task/task_traits.h"
+#include "base/task/thread_pool.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
 #include "chrome/browser/about_flags.h"
@@ -615,11 +616,11 @@ void ChromeBrowserMainExtraPartsMetrics::PostBrowserStart() {
                             UMA_LINUX_WINDOW_MANAGER_COUNT);
 #endif
 
-  constexpr base::TaskTraits background_task_traits = {
-      base::ThreadPool(), base::MayBlock(), base::TaskPriority::BEST_EFFORT,
+  constexpr base::TaskTraits kBestEffortTaskTraits = {
+      base::MayBlock(), base::TaskPriority::BEST_EFFORT,
       base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN};
 #if defined(OS_LINUX) && !defined(OS_CHROMEOS)
-  base::PostTask(FROM_HERE, background_task_traits,
+  base::ThreadPool::PostTask(FROM_HERE, kBestEffortTaskTraits,
                              base::BindOnce(&RecordLinuxDistro));
 #endif
 
@@ -644,10 +645,10 @@ void ChromeBrowserMainExtraPartsMetrics::PostBrowserStart() {
 #if defined(OS_WIN)
   // RecordStartupMetrics calls into shell_integration::GetDefaultBrowser(),
   // which requires a COM thread on Windows.
-  base::CreateCOMSTATaskRunner(background_task_traits)
+  base::ThreadPool::CreateCOMSTATaskRunner(kBestEffortTaskTraits)
       ->PostTask(FROM_HERE, base::BindOnce(&RecordStartupMetrics));
 #else
-  base::PostTask(FROM_HERE, background_task_traits,
+  base::ThreadPool::PostTask(FROM_HERE, kBestEffortTaskTraits,
                              base::BindOnce(&RecordStartupMetrics));
 #endif  // defined(OS_WIN)
 
@@ -656,7 +657,7 @@ void ChromeBrowserMainExtraPartsMetrics::PostBrowserStart() {
   // breaking some tests, including all of the ProcessMemoryMetricsEmitterTest
   // tests. Figure out why there is a dependency and fix the tests.
   auto background_task_runner =
-      base::CreateSequencedTaskRunner(background_task_traits);
+      base::ThreadPool::CreateSequencedTaskRunner(kBestEffortTaskTraits);
 
   background_task_runner->PostDelayedTask(
       FROM_HERE, base::BindOnce(&RecordIsPinnedToTaskbarHistogram),