cc: Add TaskGraphRunner sub-namespaces with task concurrency limits.

cc/resources/tile_task_worker_pool.h

Index: cc/resources/tile_task_worker_pool.h
diff --git a/cc/resources/tile_task_worker_pool.h b/cc/resources/tile_task_worker_pool.h
index 915a3fa0a8ca2afe9e07644fb13e6cb906a40323..4b2dc65af348d6cd3bee9933535a9d4c636e0221 100644
--- a/cc/resources/tile_task_worker_pool.h
+++ b/cc/resources/tile_task_worker_pool.h
@@ -20,6 +20,9 @@ class RenderingStatsInstrumentation;
 
 class CC_EXPORT TileTaskWorkerPool {
  public:
+  static const int kDefaultSubNamespace = 0;
+  static const int kDefaultMaxConcurrentTasks = 16;
+
   static unsigned kBenchmarkTaskPriority;
   static unsigned kTaskSetFinishedTaskPriorityBase;
   static unsigned kTileTaskPriorityBase;
@@ -58,7 +61,9 @@ class CC_EXPORT TileTaskWorkerPool {
   static void InsertNodeForTask(TaskGraph* graph,
                                 TileTask* task,
                                 unsigned priority,
-                                size_t dependencies);
+                                size_t dependencies,
+                                int sub_namespace,
+                                int max_concurrent_tasks);
 
   // Utility function that can be used to build a task graph. Inserts nodes that
   // represent |task| and all its image decode dependencies in |graph|.
@@ -66,7 +71,9 @@ class CC_EXPORT TileTaskWorkerPool {
       TaskGraph* graph,
       RasterTask* task,
       const ImageDecodeTask::Vector& decode_tasks,
-      unsigned priority);
+      unsigned priority,
+      int sub_namespace,
+      int max_concurrent_tasks);
 
   // Utility function that will create a temporary bitmap and copy pixels to
   // |memory| when necessary.