cc: Simplify task and its derived classes.

cc/raster/synchronous_task_graph_runner.cc

Index: cc/raster/synchronous_task_graph_runner.cc
diff --git a/cc/raster/synchronous_task_graph_runner.cc b/cc/raster/synchronous_task_graph_runner.cc
index 2f5055d87b163287c0fa73682135ef771577a5d0..0cebcfab8de9b0c748f089d204686bbefbc0c3fd 100644
--- a/cc/raster/synchronous_task_graph_runner.cc
+++ b/cc/raster/synchronous_task_graph_runner.cc
@@ -54,7 +54,7 @@ void SynchronousTaskGraphRunner::WaitForTasksToFinishRunning(
 
 void SynchronousTaskGraphRunner::CollectCompletedTasks(
     NamespaceToken token,
-    Task::Vector* completed_tasks) {
+    DependencyTask::Vector* completed_tasks) {
   TRACE_EVENT0("cc", "SynchronousTaskGraphRunner::CollectCompletedTasks");
 
   DCHECK(token.IsValid());
@@ -84,7 +84,7 @@ bool SynchronousTaskGraphRunner::RunTask() {
   const uint16_t category = found->first;
   auto prioritized_task = work_queue_.GetNextTaskToRun(category);
 
-  Task* task = prioritized_task.task;
+  DependencyTask* task = prioritized_task.task;
   task->WillRun();
   task->RunOnWorkerThread();
   task->DidRun();