ash: 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: I7647e425e2d58b5b23df3cc7bb391665f1c87902
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2063010
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742594}

ash/perftests/overview_animations_preftest.cc

@@ -7,7 +7,7 @@
 #include "base/macros.h"
 #include "base/run_loop.h"
 #include "base/system/sys_info.h"
-#include "base/task/post_task.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "ui/base/test/ui_controls.h"
 
 // Test overview enter/exit animations with following conditions
@@ -46,8 +46,9 @@ class OverviewAnimationsTest
     int wait_seconds = (base::SysInfo::IsRunningOnChromeOS() ? 5 : 0) +
                        n_browsers * cost_per_browser;
     base::RunLoop run_loop;
-    base::PostDelayedTask(FROM_HERE, run_loop.QuitClosure(),
-                          base::TimeDelta::FromSeconds(wait_seconds));
+    base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
+        FROM_HERE, run_loop.QuitClosure(),
+        base::TimeDelta::FromSeconds(wait_seconds));
     run_loop.Run();
   }