Remove unused variable OcclusionTracker::prevent_occlusion.

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index c17c3b28f2de2829bde27741f0a9db4c1ba025c7..347017556b1fb8f513c7abc790d0b2f5f5194e00 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -1033,13 +1033,12 @@ void LayerTreeHost::PaintLayerContents(
 
   in_paint_layer_contents_ = true;
 
-  const bool prevent_occlusion = false;
   LayerIteratorType end = LayerIteratorType::End(&render_surface_layer_list);
   for (LayerIteratorType it =
            LayerIteratorType::Begin(&render_surface_layer_list);
        it != end;
        ++it) {
-    occlusion_tracker.EnterLayer(it, prevent_occlusion);
+    occlusion_tracker.EnterLayer(it);
 
     if (it.represents_target_render_surface()) {
       PaintMasksForRenderSurface(