Clean up task tracker.

This cleans up some code introduced in
https://chromium-review.googlesource.com/c/chromium/src/+/1678937.

Bug: 978888
Change-Id: Ic66516d58e379b7cadc2da9d0fffede8d5c35126
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1732748Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683786}

base/task/task_traits.h

@@ -66,8 +66,6 @@ enum class TaskPriority : uint8_t {
   HIGHEST = USER_BLOCKING
 };
 
-using TaskPriorityType = std::underlying_type<TaskPriority>::type;
-
 // Valid shutdown behaviors supported by the thread pool.
 enum class TaskShutdownBehavior : uint8_t {
   // Tasks posted with this mode which have not started executing before

base/task/thread_pool/task_tracker.cc

@@ -308,7 +308,7 @@ TaskTracker::TaskTracker(StringPiece histogram_label)
       tracked_ref_factory_(this) {
   // Confirm that all |task_latency_histograms_| have been initialized above.
   for (TaskPriorityType i = 0; i < kNumTaskPriorities; ++i) {
-    for (TaskPriorityType j = 0; j < kNumBlockingModes; ++j) {
+    for (uint8_t j = 0; j < kNumBlockingModes; ++j) {
       DCHECK(task_latency_histograms_[i][j]);
     }
   }

base/task/thread_pool/task_tracker.h

@@ -285,9 +285,10 @@ class BASE_EXPORT TaskTracker {
   // blocking tasks. Intentionally leaked.
   // TODO(scheduler-dev): Consider using STATIC_HISTOGRAM_POINTER_GROUP for
   // these.
-  static constexpr auto kNumTaskPriorities =
+  using TaskPriorityType = std::underlying_type<TaskPriority>::type;
+  static constexpr TaskPriorityType kNumTaskPriorities =
       static_cast<TaskPriorityType>(TaskPriority::HIGHEST) + 1;
-  static constexpr TaskPriorityType kNumBlockingModes = 2;
+  static constexpr uint8_t kNumBlockingModes = 2;
   HistogramBase* const task_latency_histograms_[kNumTaskPriorities]
                                                [kNumBlockingModes];
   HistogramBase* const heartbeat_latency_histograms_[kNumTaskPriorities]