content: 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: I74434ddac6214f954b040adb8eec904549fc725f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062338
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742686}

content/browser/accessibility/browser_accessibility_mac.mm

@@ -6,7 +6,7 @@
 
 #import "content/browser/accessibility/browser_accessibility_mac.h"
 
-#include "base/task/post_task.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "base/time/time.h"
 #import "content/browser/accessibility/browser_accessibility_cocoa.h"
 #include "content/browser/accessibility/browser_accessibility_manager_mac.h"
@@ -87,7 +87,7 @@ void BrowserAccessibilityMac::ReplaceNativeObject() {
   base::scoped_nsobject<BrowserAccessibilityCocoa> retained_destroyed_node(
       [old_native_obj retain]);
 
-  base::PostDelayedTask(
+  base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
       FROM_HERE,
       base::BindOnce(
           [](base::scoped_nsobject<BrowserAccessibilityCocoa> destroyed) {

content/public/test/browser_test_base.cc

@@ -31,6 +31,7 @@
 #include "base/test/scoped_run_loop_timeout.h"
 #include "base/test/test_timeouts.h"
 #include "base/threading/thread_restrictions.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "content/browser/browser_main_loop.h"
 #include "content/browser/browser_thread_impl.h"
@@ -560,7 +561,7 @@ void BrowserTestBase::WaitUntilJavaIsReady(base::OnceClosure quit_closure) {
     return;
   }
 
-  base::PostDelayedTask(
+  base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
       FROM_HERE,
       base::BindOnce(&BrowserTestBase::WaitUntilJavaIsReady,
                      base::Unretained(this), std::move(quit_closure)),