Remove unused variable OcclusionTracker::prevent_occlusion.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 7f7d45a72dfa860111b4ed72d4ad9d19ed2aa1e1..6f267cc51b0417ac65f35299e15a4be158db4da9 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -749,7 +749,6 @@ bool LayerTreeHostImpl::CalculateRenderPasses(FrameData* frame) {
 
   const DrawMode draw_mode = GetDrawMode(output_surface_.get());
 
-  const bool prevent_occlusion = false;
   LayerIteratorType end =
       LayerIteratorType::End(frame->render_surface_layer_list);
   for (LayerIteratorType it =
@@ -761,7 +760,7 @@ bool LayerTreeHostImpl::CalculateRenderPasses(FrameData* frame) {
     RenderPass* target_render_pass =
         frame->render_passes_by_id[target_render_pass_id];
 
-    occlusion_tracker.EnterLayer(it, prevent_occlusion);
+    occlusion_tracker.EnterLayer(it);
 
     AppendQuadsData append_quads_data(target_render_pass_id);