cc: Always check for completed raster tasks before scheduling more tasks.

cc/resources/image_raster_worker_pool.cc

 // Copyright 2013 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 "cc/resources/image_raster_worker_pool.h"
 
 #include "base/debug/trace_event.h"
 #include "base/values.h"
 #include "cc/debug/traced_value.h"
 #include "cc/resources/resource.h"
@@ skipping 82 matching lines @@
   scoped_refptr<internal::WorkerPoolTask> new_raster_finished_task(
       CreateRasterFinishedTask());
   internal::GraphNode* raster_finished_node =
       CreateGraphNodeForTask(new_raster_finished_task.get(),
                              priority++,
                              &graph);
 
   for (RasterTaskVector::const_iterator it = raster_tasks().begin();
        it != raster_tasks().end(); ++it) {
     internal::RasterWorkerPoolTask* task = it->get();
+    DCHECK(!task->HasCompleted());
+    DCHECK(!task->WasCanceled());
 
     TaskMap::iterator image_it = image_tasks_.find(task);
     if (image_it != image_tasks_.end()) {
       internal::WorkerPoolTask* image_task = image_it->second.get();
       CreateGraphNodeForImageTask(
           image_task,
           task->dependencies(),
           priority++,
           IsRasterTaskRequiredForActivation(task),
           raster_required_for_activation_finished_node,
@@ skipping 105 matching lines @@
   if (is_required_for_activation) {
     raster_required_for_activation_finished_node->add_dependency();
     image_node->add_dependent(raster_required_for_activation_finished_node);
   }
 
   raster_finished_node->add_dependency();
   image_node->add_dependent(raster_finished_node);
 }
 
 }  // namespace cc