TaskScheduler: Do not limit the number of BLOCK_SHUTDOWN tasks posted during shutdown.

There is currently a CHECK that no more than 1000 BLOCK_SHUTDOWN tasks
are posted during shutdown. The goal of this CHECK is to prevent
shutdown hangs caused by components that behave badly (e.g. a component
that reposts a BLOCK_SHUTDOWN task at the end of each BLOCK_SHUTDOWN
task).

This CL removes this CHECK because:
- It is redundant with the hang watcher.
- Unlike the failures reported by the hang watcher, the failures
  reported by this CHECK aren't necessarily associated with user pain
  (e.g. posting more than 1000 very short BLOCK_SHUTDOWN tasks during
  shutdown will probably not be noticed by the user if it doesn't
  trigger the hang watcher).

Bug: 897334
Change-Id: I6c0ce508c5bd892724a92add3025afe9019a35e7
Reviewed-on: https://chromium-review.googlesource.com/c/1318173Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605461}

base/task/task_scheduler/task_tracker.cc

@@ -133,12 +133,6 @@ HistogramBase* GetHistogramForTaskTraits(
                         : 0];
 }
 
-// Maximum number of BLOCK_SHUTDOWN tasks that can be posted during shutdown. If
-// that many BLOCK_SHUTDOWN tasks are posted during shutdown, it is possible
-// that buggy code is posting an infinite number of tasks and that shutdown will
-// never complete. The mitigation is to induce a crash.
-constexpr int kMaxBlockShutdownTasksPostedDuringShutdown = 1000;
-
 // Returns the maximum number of TaskPriority::BEST_EFFORT sequences that can be
 // scheduled concurrently based on command line flags.
 int GetMaxNumScheduledBestEffortSequences() {
@@ -663,7 +657,6 @@ void TaskTracker::PerformShutdown() {
 
     // This method can only be called once.
     DCHECK(!shutdown_event_);
-    DCHECK(!num_block_shutdown_tasks_posted_during_shutdown_);
     DCHECK(!state_->HasShutdownStarted());
 
     shutdown_event_ = std::make_unique<WaitableEvent>();
@@ -783,10 +776,6 @@ bool TaskTracker::BeforePostTask(
         state_->DecrementNumTasksBlockingShutdown();
         return false;
       }
-
-      ++num_block_shutdown_tasks_posted_during_shutdown_;
-      CHECK_LT(num_block_shutdown_tasks_posted_during_shutdown_,
-               kMaxBlockShutdownTasksPostedDuringShutdown);
     }
 
     return true;

base/task/task_scheduler/task_tracker.h

@@ -396,9 +396,6 @@ class BASE_EXPORT TaskTracker {
   bool execution_fence_enabled_ = false;
 #endif
 
-  // Number of BLOCK_SHUTDOWN tasks posted during shutdown.
-  HistogramBase::Sample num_block_shutdown_tasks_posted_during_shutdown_ = 0;
-
   // Enforces that |max_scheduled_sequences| and
   // |max_scheduled_sequences_before_fence| in PreemptedState are only written
   // on the main sequence (determined by the first call to