cc: Fix regression in TileManager InsertNodeForDecodeTask().

cc/tiles/tile_manager.cc

 // Copyright 2012 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/tiles/tile_manager.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 141 matching lines @@
                       }) == graph->nodes.end());
   graph->nodes.push_back(
       TaskGraph::Node(task, category, priority, dependencies));
 }
 
 void InsertNodeForDecodeTask(TaskGraph* graph,
                              TileTask* task,
                              uint16_t category,
                              uint16_t priority) {
   uint32_t dependency_count = 0u;
   if (task->dependencies().size()) {

[prashant.n, 2016/04/21 12:19:34]

ericrk@, should we create a node for imageuploadtaskimpl, if image is already
uploaded??

     DCHECK_EQ(task->dependencies().size(), 1u);
     auto* dependency = task->dependencies()[0].get();
+    DCHECK(dependency)
+        << "Task |dependencies_| should hold valid dependencies.";
     if (!dependency->HasCompleted()) {
       InsertNodeForDecodeTask(graph, dependency, category, priority);
       graph->edges.push_back(TaskGraph::Edge(dependency, task));
       dependency_count = 1u;
     }
   }
   InsertNodeForTask(graph, task, task->supports_concurrent_execution()
                                      ? category
                                      : TASK_CATEGORY_NONCONCURRENT_FOREGROUND,
                     priority, dependency_count);
@@ skipping 1063 matching lines @@
 void TileManager::Signals::reset() {
   ready_to_activate = false;
   did_notify_ready_to_activate = false;
   ready_to_draw = false;
   did_notify_ready_to_draw = false;
   all_tile_tasks_completed = false;
   did_notify_all_tile_tasks_completed = false;
 }
 
 }  // namespace cc