Allow task pools to reason about transparency.

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 740555f9f442773bbfe00af2525ebdbd9aa219b5..b7b6b82eade51546e5eb23ed4466b629b7a6b187 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -642,6 +642,9 @@ ScopedTilePtr PictureLayerImpl::CreateTile(const Tile::CreateInfo& info) {
   if (!is_mask_)
     flags = Tile::USE_PICTURE_ANALYSIS;
 
+  if (contents_opaque())
+    flags |= Tile::IS_OPAQUE;
+
   return layer_tree_impl()->tile_manager()->CreateTile(
       info, id(), layer_tree_impl()->source_frame_number(), flags);
 }