gpu: Add flag for enabling asynchronous worker context.

cc/trees/layer_tree_settings.h

Index: cc/trees/layer_tree_settings.h
diff --git a/cc/trees/layer_tree_settings.h b/cc/trees/layer_tree_settings.h
index 34b1b8b71661e63412cc4d701c2ea39c7b7f4458..0e1fd60e8919286f5cab4089c6d1cf1276f01167 100644
--- a/cc/trees/layer_tree_settings.h
+++ b/cc/trees/layer_tree_settings.h
@@ -48,6 +48,7 @@ class CC_EXPORT LayerTreeSettings {
   bool use_distance_field_text;
   bool gpu_rasterization_enabled;
   bool gpu_rasterization_forced;
+  bool async_worker_context_enabled;
   int gpu_rasterization_msaa_sample_count;
   float gpu_rasterization_skewport_target_time_in_seconds;
   bool create_low_res_tiling;