cc: Add threaded GPU rasterization.

cc/layers/picture_layer_impl.cc

Index: cc/layers/picture_layer_impl.cc
diff --git a/cc/layers/picture_layer_impl.cc b/cc/layers/picture_layer_impl.cc
index dbe343d08446b986f33a688e4b3b0c2c5a251508..f3fadb34837424f4081e18b74e24c1cb7398624d 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -1136,11 +1136,12 @@ float PictureLayerImpl::MaximumTilingContentsScale() const {
 scoped_ptr<PictureLayerTilingSet>
 PictureLayerImpl::CreatePictureLayerTilingSet() {
   const LayerTreeSettings& settings = layer_tree_impl()->settings();
+  float skewport_factor = layer_tree_impl()->use_gpu_rasterization()
+                              ? settings.gpu_rasterization_skewport_factor
+                              : 1.0f;
   return PictureLayerTilingSet::Create(
       this, settings.max_tiles_for_interest_area,
-      layer_tree_impl()->use_gpu_rasterization()
-          ? 0.f
-          : settings.skewport_target_time_in_seconds,
+      settings.skewport_target_time_in_seconds * skewport_factor,
       settings.skewport_extrapolation_limit_in_content_pixels);
 }