Move and rename TaskTimeTracker to public interface exposed to WebThread, use in WebPerf Agent.

third_party/WebKit/Source/platform/scheduler/base/long_task_tracker.cc

-// Copyright 2016 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include "platform/scheduler/base/long_task_tracker.h"
-
-namespace blink {
-namespace scheduler {
-
-namespace {
-int kLongTaskThresholdMillis = 50;
-}
-
-LongTaskTracker::LongTaskTracker() {}
-
-LongTaskTracker::~LongTaskTracker() {}
-
-void LongTaskTracker::RecordLongTask(base::TimeTicks startTime,
-                                     base::TimeDelta duration) {
-  if (duration.InMilliseconds() > kLongTaskThresholdMillis) {
-    long_task_times_.push_back(std::make_pair(startTime, duration));
-  }
-}
-
-LongTaskTracker::LongTaskTiming LongTaskTracker::GetLongTaskTiming() {
-  return std::move(long_task_times_);
-}
-
-}  // namespace scheduler
-}  // namespace blink