TaskScheduler: Record TaskScheduler.NumTasksBeforeDetach.* from OnDetach().

base/task_scheduler/scheduler_worker.cc

Index: base/task_scheduler/scheduler_worker.cc
diff --git a/base/task_scheduler/scheduler_worker.cc b/base/task_scheduler/scheduler_worker.cc
index e64ca61de00600db5cc08b2d7b7937f742db860f..054227193bbad5a82c3bbe6830736b80d9919dbb 100644
--- a/base/task_scheduler/scheduler_worker.cc
+++ b/base/task_scheduler/scheduler_worker.cc
@@ -36,10 +36,7 @@ class SchedulerWorker::Thread : public PlatformThread::Delegate {
     // Set if this thread was detached.
     std::unique_ptr<Thread> detached_thread;
 
-    outer_->delegate_->OnMainEntry(
-        outer_, outer_->last_detach_time_.is_null()
-                    ? TimeDelta::Max()
-                    : TimeTicks::Now() - outer_->last_detach_time_);
+    outer_->delegate_->OnMainEntry(outer_);
 
     // A SchedulerWorker starts out waiting for work.
     WaitForWork();
@@ -60,9 +57,9 @@ class SchedulerWorker::Thread : public PlatformThread::Delegate {
         if (outer_->delegate_->CanDetach(outer_)) {
           detached_thread = outer_->Detach();
           if (detached_thread) {
+            outer_ = nullptr;
             DCHECK_EQ(detached_thread.get(), this);
             PlatformThread::Detach(thread_handle_);
-            outer_ = nullptr;
             break;
           }
         }
@@ -256,7 +253,11 @@ std::unique_ptr<SchedulerWorker::Thread> SchedulerWorker::Detach() {
   // guarantee that we call GetWork() after a successful wakeup.
   if (thread_->IsWakeUpPending())
     return nullptr;
-  last_detach_time_ = TimeTicks::Now();
+
+  // Call OnDetach() within the scope of |thread_lock_| to prevent the delegate
+  // from being used concurrently from an old and a new thread.
+  delegate_->OnDetach();
+
   return std::move(thread_);
 }