[base] Make PostTaskAndReply destination explicit in /components/arc/intent_helper

Task APIs v3 will all have an explicit destination. In the past
"default traits" meant ThreadPool. This CL is thus a no-op.

These changes are required to phase out task APIs v2 (post_task.h)
Design doc: https://docs.google.com/document/d/1tssusPykvx3g0gvbvU4HxGyn3MjJlIylnsH13-Tv6s4/edit

This CL was uploaded by git cl split.

R=djacobo@chromium.org

Bug: 1026641, 968047
Change-Id: I0009714cea5659d154e7f278b18499e41ff745fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2219169
Commit-Queue: David Jacobo <djacobo@chromium.org>
Reviewed-by: David Jacobo <djacobo@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772494}

components/arc/intent_helper/activity_icon_loader.cc

@@ -13,6 +13,7 @@
 #include "base/bind.h"
 #include "base/memory/ref_counted.h"
 #include "base/task/post_task.h"
+#include "base/task/thread_pool.h"
 #include "components/arc/arc_service_manager.h"
 #include "components/arc/arc_util.h"
 #include "components/arc/session/arc_bridge_service.h"
@@ -252,7 +253,7 @@ void ActivityIconLoader::OnIconsReady(
     OnIconsReadyCallback cb,
     std::vector<mojom::ActivityIconPtr> icons) {
   DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
-  base::PostTaskAndReplyWithResult(
+  base::ThreadPool::PostTaskAndReplyWithResult(
       FROM_HERE,
       base::BindOnce(&ResizeAndEncodeIcons, std::move(icons), scale_factor_),
       base::BindOnce(&ActivityIconLoader::OnIconsResized,