Migrate off base::ThreadPool()-as-a-trait in /gin

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=rmcilroy@chromium.org

Bug: 1026641
Change-Id: Ic1a1d402ae94bee668ddb7f449b0a57f1004f6e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079075
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745513}

gin/v8_platform.cc

@@ -31,13 +31,13 @@ namespace {
 base::LazyInstance<V8Platform>::Leaky g_v8_platform = LAZY_INSTANCE_INITIALIZER;
 
 constexpr base::TaskTraits kLowPriorityTaskTraits = {
-    base::ThreadPool(), base::TaskPriority::BEST_EFFORT};
+    base::TaskPriority::BEST_EFFORT};
 
 constexpr base::TaskTraits kDefaultTaskTraits = {
-    base::ThreadPool(), base::TaskPriority::USER_VISIBLE};
+    base::TaskPriority::USER_VISIBLE};
 
 constexpr base::TaskTraits kBlockingTaskTraits = {
-    base::ThreadPool(), base::TaskPriority::USER_BLOCKING};
+    base::TaskPriority::USER_BLOCKING};
 
 void PrintStackTrace() {
   base::debug::StackTrace trace;
@@ -417,27 +417,28 @@ int V8Platform::NumberOfWorkerThreads() {
 }
 
 void V8Platform::CallOnWorkerThread(std::unique_ptr<v8::Task> task) {
-  base::PostTask(FROM_HERE, kDefaultTaskTraits,
-                 base::BindOnce(&v8::Task::Run, std::move(task)));
+  base::ThreadPool::PostTask(FROM_HERE, kDefaultTaskTraits,
+                             base::BindOnce(&v8::Task::Run, std::move(task)));
 }
 
 void V8Platform::CallBlockingTaskOnWorkerThread(
     std::unique_ptr<v8::Task> task) {
-  base::PostTask(FROM_HERE, kBlockingTaskTraits,
-                 base::BindOnce(&v8::Task::Run, std::move(task)));
+  base::ThreadPool::PostTask(FROM_HERE, kBlockingTaskTraits,
+                             base::BindOnce(&v8::Task::Run, std::move(task)));
 }
 
 void V8Platform::CallLowPriorityTaskOnWorkerThread(
     std::unique_ptr<v8::Task> task) {
-  base::PostTask(FROM_HERE, kLowPriorityTaskTraits,
-                 base::BindOnce(&v8::Task::Run, std::move(task)));
+  base::ThreadPool::PostTask(FROM_HERE, kLowPriorityTaskTraits,
+                             base::BindOnce(&v8::Task::Run, std::move(task)));
 }
 
 void V8Platform::CallDelayedOnWorkerThread(std::unique_ptr<v8::Task> task,
                                            double delay_in_seconds) {
-  base::PostDelayedTask(FROM_HERE, kDefaultTaskTraits,
-                        base::BindOnce(&v8::Task::Run, std::move(task)),
-                        base::TimeDelta::FromSecondsD(delay_in_seconds));
+  base::ThreadPool::PostDelayedTask(
+      FROM_HERE, kDefaultTaskTraits,
+      base::BindOnce(&v8::Task::Run, std::move(task)),
+      base::TimeDelta::FromSecondsD(delay_in_seconds));
 }
 
 bool V8Platform::IdleTasksEnabled(v8::Isolate* isolate) {