Revert "Migrate off base::ThreadPool()-as-a-trait in /chrome/browser/apps"

This reverts commit a8436a80.

Reason for revert: This looks highly related to flakiness failure of MusicManagerPrivateTest.DeviceIdValueReturned on Mac builders. Let me revert this specualtively.

Original change's description:
> Migrate off base::ThreadPool()-as-a-trait in /chrome/browser/apps
> 
> 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=​dominickn@chromium.org
> 
> Bug: 1026641
> Change-Id: I459605163ef0c2a48b8e6e803721c5a16dd80ef6
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078658
> Commit-Queue: Gabriel Charette <gab@chromium.org>
> Auto-Submit: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Dominick Ng <dominickn@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#745669}

TBR=gab@chromium.org,dominickn@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1026641, 1057502
Change-Id: I166a8cacd4225787ca752577d8f1af3abb9fe117
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2081716Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745831}

chrome/browser/apps/platform_apps/api/music_manager_private/device_id.h

@@ -44,13 +44,15 @@ class DeviceId {
   // new MAC addresses at each reboot.
   static bool IsValidMacAddress(const void* bytes, size_t size);
 
-  // The traits of the task on base::ThreadPool that retrieves the device id.
+  // The traits of the task that retrieves the device id.
   //
+  // ThreadPool(): This should run on a background thread.
   // MayBlock(): Since this requires fetching disk.
   // TaskPriority: USER_VISIBLE. Though this might be conservative, depending
   //   on how GetDeviceId() is used.
   static constexpr base::TaskTraits traits() {
-    return {base::MayBlock(), base::TaskPriority::USER_VISIBLE};
+    return {base::ThreadPool(), base::MayBlock(),
+            base::TaskPriority::USER_VISIBLE};
   }
 };
 

chrome/browser/apps/platform_apps/api/music_manager_private/device_id_linux.cc

@@ -20,7 +20,7 @@
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
-#include "base/task/thread_pool.h"
+#include "base/task/post_task.h"
 #include "base/threading/scoped_blocking_call.h"
 #include "content/public/browser/browser_task_traits.h"
 #include "content/public/browser/browser_thread.h"
@@ -182,8 +182,8 @@ void GetRawDeviceIdImpl(const IsValidMacAddressCallback& is_valid_mac_address,
     raw_device_id = mac_address + disk_id;
   }
 
-  content::GetUIThreadTaskRunner({})->PostTask(
-      FROM_HERE, base::BindOnce(callback, raw_device_id));
+  base::PostTask(FROM_HERE, {content::BrowserThread::UI},
+                 base::BindOnce(callback, raw_device_id));
 }
 
 }  // namespace
@@ -192,7 +192,7 @@ void GetRawDeviceIdImpl(const IsValidMacAddressCallback& is_valid_mac_address,
 void DeviceId::GetRawDeviceId(const IdCallback& callback) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
 
-  base::ThreadPool::PostTask(
+  base::PostTask(
       FROM_HERE, traits(),
       base::BindOnce(&GetRawDeviceIdImpl,
                      base::Bind(&DeviceId::IsValidMacAddress), callback));

chrome/browser/apps/platform_apps/api/music_manager_private/device_id_win.cc

@@ -23,7 +23,7 @@
 #include "base/scoped_native_library.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
-#include "base/task/thread_pool.h"
+#include "base/task/post_task.h"
 #include "base/threading/scoped_blocking_call.h"
 #include "base/win/windows_version.h"
 #include "content/public/browser/browser_task_traits.h"
@@ -176,8 +176,8 @@ void GetMacAddress(const IsValidMacAddressCallback& is_valid_mac_address,
     LOG(ERROR) << "Could not find appropriate MAC address.";
   }
 
-  content::GetUIThreadTaskRunner({})->PostTask(
-      FROM_HERE, base::BindOnce(callback, mac_address));
+  base::PostTask(FROM_HERE, {content::BrowserThread::UI},
+                 base::BindOnce(callback, mac_address));
 }
 
 std::string GetRlzMachineId() {
@@ -209,7 +209,7 @@ void GetMacAddressCallback(const DeviceId::IdCallback& callback,
 void DeviceId::GetRawDeviceId(const IdCallback& callback) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
 
-  base::ThreadPool::PostTask(
+  base::PostTask(
       FROM_HERE, traits(),
       base::Bind(&GetMacAddress, base::Bind(&DeviceId::IsValidMacAddress),
                  base::Bind(&GetMacAddressCallback, callback)));