Re-land: cc: Add raster finished signals to RasterWorkerPool.

cc/resources/worker_pool.cc

Index: cc/resources/worker_pool.cc
diff --git a/cc/resources/worker_pool.cc b/cc/resources/worker_pool.cc
index 90e30328cec071092fd60976e27c04aff408a3b3..8080dcf18c2e380072ef50aab834b9d7d42ca5c6 100644
--- a/cc/resources/worker_pool.cc
+++ b/cc/resources/worker_pool.cc
@@ -363,9 +363,10 @@ void WorkerPool::Inner::Run() {
   has_ready_to_run_tasks_cv_.Signal();
 }
 
-WorkerPool::GraphNode::GraphNode()
-    : task_(NULL),
-      priority_(0),
+WorkerPool::GraphNode::GraphNode(
+    internal::WorkerPoolTask* task, unsigned priority)
+    : task_(task),
+      priority_(priority),
       num_dependencies_(0) {
 }