Minor cleanup in Blink Scheduler.

Removed outdated TODO.
Removed unnecessary use of ThreadTaskRunnerHandle.

Change-Id: Ib9f36222c6fd8745d94177fe0762188fdd551fa9
Reviewed-on: https://chromium-review.googlesource.com/1102327
Commit-Queue: Greg Kraynov <kraynov@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567606}

base/task/sequence_manager/enqueue_order.h

@@ -42,7 +42,6 @@ class EnqueueOrder {
     ~Generator();
 
     // Can be called from any thread.
-    // TODO(scheduler-dev): Is it the right atomic? https://crbug.com/852344.
     EnqueueOrder GenerateNext() {
       return EnqueueOrder(std::atomic_fetch_add_explicit(
           &counter_, uint64_t(1), std::memory_order_relaxed));

third_party/blink/renderer/platform/scheduler/common/scheduler_helper_unittest.cc

@@ -10,7 +10,6 @@
 #include "base/single_thread_task_runner.h"
 #include "base/task/sequence_manager/lazy_now.h"
 #include "base/test/scoped_task_environment.h"
-#include "base/threading/thread_task_runner_handle.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/blink/renderer/platform/scheduler/base/task_queue_forward.h"
@@ -58,7 +57,7 @@ class SchedulerHelperTest : public testing::Test {
     std::unique_ptr<base::sequence_manager::TaskQueueManagerForTest>
         task_queue_manager =
             base::sequence_manager::TaskQueueManagerForTest::Create(
-                nullptr, base::ThreadTaskRunnerHandle::Get(),
+                nullptr, task_environment_.GetMainThreadTaskRunner(),
                 task_environment_.GetMockTickClock());
     task_queue_manager_ = task_queue_manager.get();
     scheduler_helper_ = std::make_unique<NonMainThreadSchedulerHelper>(

third_party/blink/renderer/platform/scheduler/main_thread/auto_advancing_virtual_time_domain_unittest.cc

@@ -7,7 +7,6 @@
 #include <memory>
 #include "base/run_loop.h"
 #include "base/test/test_mock_time_task_runner.h"
-#include "base/threading/thread_task_runner_handle.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/blink/renderer/platform/scheduler/base/task_queue_manager.h"
@@ -34,8 +33,7 @@ class AutoAdvancingVirtualTimeDomainTest : public testing::Test {
         base::TimeDelta::FromMilliseconds(5));
     scheduler_helper_.reset(new NonMainThreadSchedulerHelper(
         base::sequence_manager::TaskQueueManagerForTest::Create(
-            nullptr, base::ThreadTaskRunnerHandle::Get(),
-            test_task_runner_->GetMockTickClock()),
+            nullptr, test_task_runner_, test_task_runner_->GetMockTickClock()),
         nullptr, TaskType::kInternalTest));
 
     scheduler_helper_->AddTaskTimeObserver(&test_task_time_observer_);

third_party/blink/renderer/platform/scheduler/worker/worker_scheduler_proxy_unittest.cc

@@ -5,7 +5,6 @@
 #include "third_party/blink/renderer/platform/scheduler/worker/worker_scheduler_proxy.h"
 #include "base/run_loop.h"
 #include "base/test/scoped_task_environment.h"
-#include "base/threading/thread_task_runner_handle.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/blink/renderer/platform/scheduler/base/test/task_queue_manager_for_test.h"
@@ -116,7 +115,7 @@ class WorkerSchedulerProxyTest : public testing::Test {
         main_thread_scheduler_(std::make_unique<MainThreadSchedulerImpl>(
             base::sequence_manager::TaskQueueManagerForTest::Create(
                 nullptr,
-                base::ThreadTaskRunnerHandle::Get(),
+                task_environment_.GetMainThreadTaskRunner(),
                 task_environment_.GetMockTickClock()),
             base::nullopt)),
         page_scheduler_(