Re-land: cc: Use worker context for one-copy tile initialization.

cc/trees/layer_tree_host_pixeltest_tiles.cc

Index: cc/trees/layer_tree_host_pixeltest_tiles.cc
diff --git a/cc/trees/layer_tree_host_pixeltest_tiles.cc b/cc/trees/layer_tree_host_pixeltest_tiles.cc
index a07f100c31a0748adf7851ff5c35f024ebbd411b..ede77dfa7affb8fec400de01fba781b37aab9092 100644
--- a/cc/trees/layer_tree_host_pixeltest_tiles.cc
+++ b/cc/trees/layer_tree_host_pixeltest_tiles.cc
@@ -161,6 +161,7 @@ class LayerTreeHostTilesTestPartialInvalidation
         // only re-raster the stuff in the rect. If it doesn't do partial raster
         // it would re-raster the whole thing instead.
         client_.set_blue_top(false);
+        Finish();
         picture_layer_->SetNeedsDisplayRect(gfx::Rect(50, 50, 100, 100));
 
         // Add a copy request to see what happened!