cc: Only create tilings for layers that draws content.

cc/picture_layer_tiling_set.cc

Index: cc/picture_layer_tiling_set.cc
diff --git a/cc/picture_layer_tiling_set.cc b/cc/picture_layer_tiling_set.cc
index f8f8ac95280f5d52931ba3a086993295baa1ef98..e435a1001c406d4a43ae85ef1f1f7dbce4d37866 100644
--- a/cc/picture_layer_tiling_set.cc
+++ b/cc/picture_layer_tiling_set.cc
@@ -48,6 +48,11 @@ void PictureLayerTilingSet::AddTiling(float contents_scale,
   tilings_.last()->SetLayerBounds(layer_bounds_);
 }
 
+void PictureLayerTilingSet::Reset() {
+  for (size_t i = 0; i < tilings_.size(); ++i)
+    tilings_[i]->Reset();
+}
+
 PictureLayerTilingSet::Iterator::Iterator(PictureLayerTilingSet* set,
                                           float contents_scale,
                                           gfx::Rect content_rect)