content: Implement dynamic priorities for raster threads.

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..4d2cb040be420ef9a90fde7bf4a135ff6bf4d6d5 100644
--- a/cc/raster/task_graph_work_queue.cc
+++ b/cc/raster/task_graph_work_queue.cc
@@ -77,6 +77,11 @@ void TaskGraphWorkQueue::ScheduleTasks(NamespaceToken token, TaskGraph* graph) {
   for (auto& ready_to_run_tasks_it : task_namespace.ready_to_run_tasks) {
     ready_to_run_tasks_it.second.clear();
   }
+
+  for (const CategorizedTask& task : task_namespace.running_tasks) {
+    task.second->cancelled = true;
+  }
+
   for (const TaskGraph::Node& node : graph->nodes) {
     // Remove any old nodes that are associated with this task. The result is
     // that the old graph is left with all nodes not present in this graph,
@@ -103,14 +108,45 @@ void TaskGraphWorkQueue::ScheduleTasks(NamespaceToken token, TaskGraph* graph) {
     if (std::any_of(task_namespace.running_tasks.begin(),
                     task_namespace.running_tasks.end(),
                     [&node](const CategorizedTask& task) {
+                      if (task.second == node.task) {
+                        if (task.first > node.category) {
+                          TRACE_EVENT2("cc, pras", "Task::Rescheduled::Speedup",
+                                       "Old Category", task.first,
+                                       "New Category", node.category);
+                        } else if (task.first < node.category) {
+                          TRACE_EVENT2("cc, pras",
+                                       "Task::Rescheduled::Slowdown",
+                                       "Old Category", task.first,
+                                       "New Category", node.category);
+                        } else {
+                          TRACE_EVENT2("cc, pras", "Task::Rescheduled::Same",
+                                       "Old Category", task.first,
+                                       "New Category", node.category);
+                        }
+
+                        task.second->cancelled = false;
+                      }
+
                       return task.second == node.task;
                     }))
       continue;
 
+    {
+      TRACE_EVENT1("cc, pras", "Task::Scheduled::New", "New Category",
+                   node.category);
+    }
+
     task_namespace.ready_to_run_tasks[node.category].push_back(PrioritizedTask(
         node.task, &task_namespace, node.category, node.priority));
   }
 
+  for (const CategorizedTask& task : task_namespace.running_tasks) {
+    if (task.second->cancelled) {
+      TRACE_EVENT1("cc, pras", "Task::Rescheduled::Cancelled", "Old Category",
+                   task.first);
+    }
+  }
+
   // Rearrange the elements in each vector within |ready_to_run_tasks| in such a
   // way that they form a heap.
   for (auto& it : task_namespace.ready_to_run_tasks) {