[base] Make PostTaskAndReply destination explicit in /components/omnibox

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=mpearson@chromium.org

Bug: 1026641, 968047
Change-Id: Ifcaee2088907f0cc8347da2f6f37259585b3ac48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2219015
Commit-Queue: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773043}

components/omnibox/browser/clipboard_provider.cc

@@ -20,7 +20,7 @@
 #include "base/optional.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/task/post_task.h"
-#include "base/task/task_traits.h"
+#include "base/task/thread_pool.h"
 #include "build/build_config.h"
 #include "components/omnibox/browser/autocomplete_input.h"
 #include "components/omnibox/browser/autocomplete_match.h"
@@ -372,7 +372,7 @@ void ClipboardProvider::OnReceiveImage(
   if (!optional_image)
     return;
   done_ = false;
-  PostTaskAndReplyWithResult(
+  base::ThreadPool::PostTaskAndReplyWithResult(
       FROM_HERE,
       base::BindOnce(&ClipboardProvider::EncodeClipboardImage,
                      optional_image.value()),