TaskGraphRunner Group support

cc/raster/task_graph_runner.h

Index: cc/raster/task_graph_runner.h
diff --git a/cc/raster/task_graph_runner.h b/cc/raster/task_graph_runner.h
index b5ffade02bdbbdbbff85ca230192c2da161bbdf9..181a455057234d636b4eca032e8d4c64443616b8 100644
--- a/cc/raster/task_graph_runner.h
+++ b/cc/raster/task_graph_runner.h
@@ -46,19 +46,29 @@ class CC_EXPORT Task : public base::RefCountedThreadSafe<Task> {
 
 // A task dependency graph describes the order in which to execute a set
 // of tasks. Dependencies are represented as edges. Each node is assigned
-// a priority and a run count that matches the number of dependencies.
-// Priority range from 0 (most favorable scheduling) to UINT_MAX
-// (least favorable).
+// a category, a priority and a run count that matches the number of
+// dependencies. Priority range from 0 (most favorable scheduling) to UINT16_MAX
+// (least favorable). Categories range from 0 to UINT16_MAX. It is up to the
+// implementation and its consumer to determine the meaning (if any) of a
+// category. A TaskGraphRunner implementation may chose to prioritize certain
+// categories over others, regardless of the individual priorities of tasks.
 struct CC_EXPORT TaskGraph {
   struct Node {
     typedef std::vector<Node> Vector;
 
-    Node(Task* task, size_t priority, size_t dependencies)
-        : task(task), priority(priority), dependencies(dependencies) {}
+    Node(Task* task,
+         uint16_t category,
+         uint16_t priority,
+         uint32_t dependencies)
+        : task(task),
+          category(category),
+          priority(priority),
+          dependencies(dependencies) {}
 
     Task* task;
-    size_t priority;
-    size_t dependencies;
+    uint16_t category;
+    uint16_t priority;
+    uint32_t dependencies;
   };
 
   struct Edge {