Add flag to enable rendering of text using signed distance fields.

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 fcb9a40ca767663513ce273c56f359834822dc58..e93503301fa11516d715123849fa939799ad8d40 100644
--- a/cc/trees/layer_tree_settings.h
+++ b/cc/trees/layer_tree_settings.h
@@ -30,6 +30,7 @@ class CC_EXPORT LayerTreeSettings {
   bool partial_swap_enabled;
   bool accelerated_animation_enabled;
   bool can_use_lcd_text;
+  bool use_distance_field_text;
   bool should_clear_root_render_pass;
   bool gpu_rasterization_enabled;
   bool gpu_rasterization_forced;