Reland - Refactor signaling in RWP

cc/raster/task_graph_work_queue.h

Index: cc/raster/task_graph_work_queue.h
diff --git a/cc/raster/task_graph_work_queue.h b/cc/raster/task_graph_work_queue.h
index 95d3c772c24f22ea478757e2d1a81d40ab231b9e..3ea984a458491aeda084118c8a5211b621890292 100644
--- a/cc/raster/task_graph_work_queue.h
+++ b/cc/raster/task_graph_work_queue.h
@@ -28,19 +28,16 @@ class CC_EXPORT TaskGraphWorkQueue {
  public:
   struct TaskNamespace;
 
-  struct PrioritizedTask {
+  struct CC_EXPORT PrioritizedTask {
     typedef std::vector<PrioritizedTask> Vector;
 
     PrioritizedTask(Task* task,
                     TaskNamespace* task_namespace,
                     uint16_t category,
-                    uint16_t priority)
-        : task(task),
-          task_namespace(task_namespace),
-          category(category),
-          priority(priority) {}
+                    uint16_t priority);
+    ~PrioritizedTask();
 
-    Task* task;
+    scoped_refptr<Task> task;

[reveman, 2016/02/24 18:53:00]

hm, not sure about this. I'm worried this will make ScheduleTasks much more
expensive as increments/decrements of these thread-safe ref-counted tasks is
very expensive compared to everything else done at ScheduleTasks time. Can we
avoid this change?

[ericrk, 2016/02/24 19:30:43]

Introduced a new type, CategorizedTask, which refcounts (as Task::Vector did),
but is only used for running_tasks. This avoids unnecessary refcounting but adds
a new type.

     TaskNamespace* task_namespace;
     uint16_t category;
     uint16_t priority;
@@ -64,7 +61,7 @@ class CC_EXPORT TaskGraphWorkQueue {
     Task::Vector completed_tasks;
 
     // This set contains all currently running tasks.
-    Task::Vector running_tasks;
+    PrioritizedTask::Vector running_tasks;
   };
 
   TaskGraphWorkQueue();
@@ -145,6 +142,19 @@ class CC_EXPORT TaskGraphWorkQueue {
     return ready_to_run_namespaces_;
   }
 
+  size_t NumRunningTasksForCategory(uint16_t category) const {
+    size_t count = 0;
+    for (const auto& task_namespace_entry : namespaces_) {
+      for (const auto& prioritized_task :
+           task_namespace_entry.second.running_tasks) {
+        if (prioritized_task.category == category) {
+          count++;
+        }
+      }
+    }
+    return count;
+  }
+
   // Helper function which ensures that graph dependencies were correctly
   // configured.
   static bool DependencyMismatch(const TaskGraph* graph);