cc: Simplify task and its derived classes.

cc/raster/task_graph_work_queue.cc

Index: cc/raster/task_graph_work_queue.cc
diff --git a/cc/raster/task_graph_work_queue.cc b/cc/raster/task_graph_work_queue.cc
index 98413eee1a668a196f137dd9f4ce0ffbfe59bdff..19a0176178dabf96de82bf02325e661b398229cd 100644
--- a/cc/raster/task_graph_work_queue.cc
+++ b/cc/raster/task_graph_work_queue.cc
@@ -65,7 +65,8 @@ void TaskGraphWorkQueue::ScheduleTasks(NamespaceToken token, TaskGraph* graph) {
   TaskNamespace& task_namespace = namespaces_[token];
 
   // First adjust number of dependencies to reflect completed tasks.
-  for (const scoped_refptr<Task>& task : task_namespace.completed_tasks) {
+  for (const scoped_refptr<DependencyTask>& task :
+       task_namespace.completed_tasks) {
     for (DependentIterator node_it(graph, task.get()); node_it; ++node_it) {
       TaskGraph::Node& node = *node_it;
       DCHECK_LT(0u, node.dependencies);
@@ -210,7 +211,7 @@ TaskGraphWorkQueue::PrioritizedTask TaskGraphWorkQueue::GetNextTaskToRun(
 
 void TaskGraphWorkQueue::CompleteTask(const PrioritizedTask& completed_task) {
   TaskNamespace* task_namespace = completed_task.task_namespace;
-  scoped_refptr<Task> task(completed_task.task);
+  scoped_refptr<DependencyTask> task(completed_task.task);
 
   // Remove task from |running_tasks|.
   auto it = std::find_if(task_namespace->running_tasks.begin(),
@@ -273,8 +274,9 @@ void TaskGraphWorkQueue::CompleteTask(const PrioritizedTask& completed_task) {
   task_namespace->completed_tasks.push_back(task);
 }
 
-void TaskGraphWorkQueue::CollectCompletedTasks(NamespaceToken token,
-                                               Task::Vector* completed_tasks) {
+void TaskGraphWorkQueue::CollectCompletedTasks(
+    NamespaceToken token,
+    DependencyTask::Vector* completed_tasks) {
   TaskNamespaceMap::iterator it = namespaces_.find(token);
   if (it == namespaces_.end())
     return;
@@ -295,7 +297,7 @@ void TaskGraphWorkQueue::CollectCompletedTasks(NamespaceToken token,
 
 bool TaskGraphWorkQueue::DependencyMismatch(const TaskGraph* graph) {
   // Value storage will be 0-initialized.
-  std::unordered_map<const Task*, size_t> dependents;
+  std::unordered_map<const DependencyTask*, size_t> dependents;
   for (const TaskGraph::Edge& edge : graph->edges)
     dependents[edge.dependent]++;