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

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=ccameron@chromium.org

Bug: 1026641
Change-Id: I92713633f2a77205ed1b4f43014fd082f8096e74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078664
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745524}

components/metal_util/test_shader.mm

@@ -778,10 +778,9 @@ void TestShader(TestShaderCallback callback,
         if (delay.is_zero()) {
           std::move(closure).Run();
         } else {
-          base::PostDelayedTask(
-              FROM_HERE,
-              base::TaskTraits(base::ThreadPool(), base::TaskPriority::HIGHEST),
-              std::move(closure), delay);
+          base::ThreadPool::PostDelayedTask(FROM_HERE,
+                                            {base::TaskPriority::HIGHEST},
+                                            std::move(closure), delay);
         }
         return;
       }