cc: Only make num_threads - 1 SkPicture clones.

cc/resources/raster_worker_pool.cc

Index: cc/resources/raster_worker_pool.cc
diff --git a/cc/resources/raster_worker_pool.cc b/cc/resources/raster_worker_pool.cc
index 30f684a31c701090cbbfc4ebc56ef5478d45e3bc..d6cbe673cfbca226dbaff41b3525344aea091809 100644
--- a/cc/resources/raster_worker_pool.cc
+++ b/cc/resources/raster_worker_pool.cc
@@ -450,6 +450,15 @@ class RasterRequiredForActivationFinishedWorkerPoolTaskImpl
       RasterRequiredForActivationFinishedWorkerPoolTaskImpl);
 };
 
+class RasterTaskGraphRunner : public internal::TaskGraphRunner {
+ public:
+  RasterTaskGraphRunner()
+      : internal::TaskGraphRunner(RasterWorkerPool::GetNumRasterThreads(),
+                                  "CompositorRaster") {}
+};
+base::LazyInstance<RasterTaskGraphRunner>::Leaky g_task_graph_runner =
+    LAZY_INSTANCE_INITIALIZER;
+
 const int kDefaultNumRasterThreads = 1;
 
 int g_num_raster_threads = 0;
@@ -554,12 +563,15 @@ void RasterWorkerPool::RasterTask::Reset() { internal_ = NULL; }
 
 RasterWorkerPool::RasterTask::~RasterTask() {}
 
+// This allows an external rasterize on-demand system to run raster tasks
+// with highest priority using the same task graph runner instance.
+unsigned RasterWorkerPool::kOnDemandRasterTaskPriority = 0u;
 // Task priorities that make sure raster finished tasks run before any
 // remaining raster tasks.
-unsigned RasterWorkerPool::kRasterFinishedTaskPriority = 1u;
+unsigned RasterWorkerPool::kRasterFinishedTaskPriority = 2u;
 unsigned RasterWorkerPool::kRasterRequiredForActivationFinishedTaskPriority =
-    0u;
-unsigned RasterWorkerPool::kRasterTaskPriorityBase = 2u;
+    1u;
+unsigned RasterWorkerPool::kRasterTaskPriorityBase = 3u;
 
 RasterWorkerPool::RasterWorkerPool(internal::TaskGraphRunner* task_graph_runner,
                                    ResourceProvider* resource_provider,
@@ -590,6 +602,11 @@ int RasterWorkerPool::GetNumRasterThreads() {
 }
 
 // static
+internal::TaskGraphRunner* RasterWorkerPool::GetTaskGraphRunner() {
+  return g_task_graph_runner.Pointer();
+}
+
+// static
 RasterWorkerPool::RasterTask RasterWorkerPool::CreateRasterTask(
     const Resource* resource,
     PicturePileImpl* picture_pile,