Remove no longer needed TileManager task references

With crrev.com/2349543003, TaskGraph::Task owns tasks, keeping them
alive while the graph is running. This means that TileManager no longer
needs to keep these tasks alive while the graph may still be using
them. Clean up these references.

R=vmpstr
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2534283002
Cr-Commit-Position: refs/heads/master@{#435454}

cc/tiles/tile_manager.cc

@@ -386,9 +386,6 @@ void TileManager::FinishTasksAndCleanUp() {
 
   tile_task_manager_->CheckForCompletedTasks();
 
-  // Now that all tasks have been finished, we can clear any |orphan_tasks_|.
-  orphan_tasks_.clear();
-
   FreeResourcesForReleasedTiles();
   CleanUpReleasedTiles();
 
@@ -949,17 +946,6 @@ void TileManager::ScheduleTasks(
   // in |raster_queue_|.
   tile_task_manager_->ScheduleTasks(&graph_);
 
-  // It's now safe to clean up orphan tasks as raster worker pool is not
-  // allowed to keep around unreferenced raster tasks after ScheduleTasks() has
-  // been called.
-  orphan_tasks_.clear();
-
-  // It's also now safe to replace our *_done_task_ tasks.
-  required_for_activation_done_task_ =
-      std::move(required_for_activation_done_task);
-  required_for_draw_done_task_ = std::move(required_for_draw_done_task);
-  all_done_task_ = std::move(all_done_task);
-
   did_check_for_completed_tasks_since_last_schedule_tasks_ = false;
 
   TRACE_EVENT_ASYNC_STEP_INTO1("cc", "ScheduledTasks", this, "running", "state",
@@ -1033,7 +1019,6 @@ void TileManager::OnRasterTaskCompleted(
 
   TileDrawInfo& draw_info = tile->draw_info();
   DCHECK(tile->raster_task_.get());
-  orphan_tasks_.push_back(tile->raster_task_);
   tile->raster_task_ = nullptr;
 
   // Unref all the images.

cc/tiles/tile_manager.h

@@ -327,12 +327,7 @@ class CC_EXPORT TileManager {
 
   std::vector<Tile*> released_tiles_;
 
-  std::vector<scoped_refptr<TileTask>> orphan_tasks_;
-
   TaskGraph graph_;
-  scoped_refptr<TileTask> required_for_activation_done_task_;
-  scoped_refptr<TileTask> required_for_draw_done_task_;
-  scoped_refptr<TileTask> all_done_task_;
 
   UniqueNotifier more_tiles_need_prepare_check_notifier_;