Migrate off base::ThreadPool()-as-a-trait in /chrome/browser/ui/tab_contents

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=avi@chromium.org

Bug: 1026641
Change-Id: I6c19f61e4bd125aab58741e0d4a98f8411f9cf0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078428
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745509}

chrome/browser/ui/tab_contents/chrome_web_contents_view_handle_drop.cc

@@ -8,6 +8,7 @@
 #include "base/files/file_util.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/task/post_task.h"
+#include "base/task/thread_pool.h"
 #include "base/task_runner_util.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/safe_browsing/cloud_content_scanning/deep_scanning_dialog_delegate.h"
@@ -100,10 +101,8 @@ void HandleOnPerformDrop(
   if (drop_data.filenames.empty()) {
     ScanData(web_contents, std::move(callback), std::move(data));
   } else {
-    base::PostTaskAndReplyWithResult(
-        FROM_HERE,
-        {base::ThreadPool(), base::TaskPriority::USER_VISIBLE,
-         base::MayBlock()},
+    base::ThreadPool::PostTaskAndReplyWithResult(
+        FROM_HERE, {base::TaskPriority::USER_VISIBLE, base::MayBlock()},
         base::BindOnce(&GetPathsToScan, web_contents, std::move(drop_data),
                        std::move(data)),
         base::BindOnce(&ScanData, web_contents, std::move(callback)));