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

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=ajwong@chromium.org

Bug: 616447
Change-Id: I0b6ecc3218f51dc3ae04f80d49ac374b33074bc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1635828
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Auto-Submit: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678343}

base/message_loop/message_loop_current.cc

@@ -51,11 +51,6 @@ void MessageLoopCurrent::RemoveDestructionObserver(
   current_->RemoveDestructionObserver(destruction_observer);
 }
 
-scoped_refptr<SingleThreadTaskRunner> MessageLoopCurrent::task_runner() const {
-  DCHECK(current_->IsBoundToCurrentThread());
-  return current_->GetTaskRunner();
-}
-
 void MessageLoopCurrent::SetTaskRunner(
     scoped_refptr<SingleThreadTaskRunner> task_runner) {
   DCHECK(current_->IsBoundToCurrentThread());

base/message_loop/message_loop_current.h

@@ -105,11 +105,6 @@ class BASE_EXPORT MessageLoopCurrent {
   // DestructionObserver is receiving a notification callback.
   void RemoveDestructionObserver(DestructionObserver* destruction_observer);
 
-  // Forwards to MessageLoop::task_runner().
-  // DEPRECATED(https://crbug.com/616447): Use ThreadTaskRunnerHandle::Get()
-  // instead of MessageLoopCurrent::Get()->task_runner().
-  scoped_refptr<SingleThreadTaskRunner> task_runner() const;
-
   // Forwards to MessageLoop::SetTaskRunner().
   // DEPRECATED(https://crbug.com/825327): only owners of the MessageLoop
   // instance should replace its TaskRunner.

base/test/scoped_mock_time_message_loop_task_runner_unittest.cc

@@ -17,6 +17,7 @@
 #include "base/message_loop/message_loop_current.h"
 #include "base/test/test_mock_time_task_runner.h"
 #include "base/test/test_pending_task.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "base/time/time.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -24,7 +25,7 @@ namespace base {
 namespace {
 
 TaskRunner* GetCurrentTaskRunner() {
-  return MessageLoopCurrent::Get()->task_runner().get();
+  return ThreadTaskRunnerHandle::Get().get();
 }
 
 void AssignTrue(bool* out) {