[MessageLoop]: Replace MessageLoopCurrent::task_runner() in /components/download.

MessageLoopCurrent::task_runner() is depecated.
This CL replaces it with with ThreadTaskRunnerHandle::Get().
A small step towards deleting MessageLoop.

This CL was generated with git cl split.
Refer to the top-level CL if necessary :
https://chromium-review.googlesource.com/c/chromium/src/+/1631693

Please CQ if LGTY!

This CL was uploaded by git cl split.

R=shaktisahu@chromium.org

Bug: 616447
Change-Id: I8a262540203b8b475cb215573b87710e9fefab0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636060
Auto-Submit: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666404}

components/download/internal/common/download_file_impl.cc

@@ -10,9 +10,9 @@
 
 #include "base/bind.h"
 #include "base/files/file_util.h"
-#include "base/message_loop/message_loop_current.h"
 #include "base/strings/stringprintf.h"
 #include "base/threading/sequenced_task_runner_handle.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "base/time/time.h"
 #include "base/timer/timer.h"
 #include "base/values.h"
@@ -167,7 +167,7 @@ DownloadFileImpl::DownloadFileImpl(
       bytes_seen_without_parallel_streams_(0),
       is_paused_(false),
       download_id_(download_id),
-      main_task_runner_(base::MessageLoopCurrent::Get()->task_runner()),
+      main_task_runner_(base::ThreadTaskRunnerHandle::Get()),
       observer_(observer),
       weak_factory_(this) {
   TRACE_EVENT_INSTANT0("download", "DownloadFileCreated",