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..aa7565473aec4c35e08d9c007db6d9c8c6d38499 100644
--- a/cc/raster/task_graph_work_queue.cc
+++ b/cc/raster/task_graph_work_queue.cc
@@ -43,6 +43,62 @@ class CompareTaskNamespacePriority {
  private:
   uint16_t category_;
 };
+
+// Helper class for iterating over all dependents of a task.
+class DependentIterator {
+ public:
+  DependentIterator(TaskGraph* graph, const Task* task)
+      : graph_(graph),
+        task_(task),
+        current_index_(static_cast<size_t>(-1)),
+        current_node_(NULL) {
+    ++(*this);
+  }
+
+  TaskGraph::Node& operator->() const {
+    DCHECK_LT(current_index_, graph_->edges.size());
+    DCHECK_EQ(graph_->edges[current_index_].task, task_);
+    DCHECK(current_node_);
+    return *current_node_;
+  }
+
+  TaskGraph::Node& operator*() const {
+    DCHECK_LT(current_index_, graph_->edges.size());
+    DCHECK_EQ(graph_->edges[current_index_].task, task_);
+    DCHECK(current_node_);
+    return *current_node_;
+  }
+
+  // Note: Performance can be improved by keeping edges sorted.
+  DependentIterator& operator++() {
+    // Find next dependency edge for |task_|.
+    do {
+      ++current_index_;
+      if (current_index_ == graph_->edges.size())
+        return *this;
+    } while (graph_->edges[current_index_].task != task_);
+
+    // Now find the node for the dependent of this edge.
+    TaskGraph::Node::Vector::iterator it = std::find_if(
+        graph_->nodes.begin(), graph_->nodes.end(),
+        [this](const TaskGraph::Node& node) {
+          return node.task == graph_->edges[current_index_].dependent;
+        });
+    DCHECK(it != graph_->nodes.end());
+    current_node_ = &(*it);
+
+    return *this;
+  }
+
+  operator bool() const { return current_index_ < graph_->edges.size(); }
+
+ private:
+  TaskGraph* graph_;
+  const Task* task_;
+  size_t current_index_;
+  TaskGraph::Node* current_node_;
+};
+
 }  // namespace
 
 TaskGraphWorkQueue::TaskNamespace::TaskNamespace() {}