apps: Make destination for base::PostDelayedTask() explicit

When posting tasks with base::PostDelayedTask(), explicitly specify whether the
tasks should run on the thread pool on a specific named thread. This is in
preparation for making it always mandatory to specify the destination for posted
tasks.

This patch has no functional changes.

This CL was uploaded by git cl split.

Bug: 968047
Change-Id: I25230a83101d68c44929f19434d38e0620ee0b95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2063009
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742575}

apps/test/app_window_waiter.cc

@@ -5,6 +5,7 @@
 #include "apps/test/app_window_waiter.h"
 
 #include "base/task/post_task.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "extensions/browser/app_window/app_window.h"
 #include "extensions/browser/app_window/native_app_window.h"
 
@@ -52,7 +53,8 @@ extensions::AppWindow* AppWindowWaiter::WaitForShownWithTimeout(
 
   wait_type_ = WAIT_FOR_SHOWN;
   run_loop_ = std::make_unique<base::RunLoop>();
-  base::PostDelayedTask(FROM_HERE, run_loop_->QuitClosure(), timeout);
+  base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
+      FROM_HERE, run_loop_->QuitClosure(), timeout);
   run_loop_->Run();
 
   return window_;