Remove unused variable OcclusionTracker::prevent_occlusion.

cc/trees/quad_culler_unittest.cc

Index: cc/trees/quad_culler_unittest.cc
diff --git a/cc/trees/quad_culler_unittest.cc b/cc/trees/quad_culler_unittest.cc
index 28987c7589fe5bd35471894d41fe405c967c018e..c07a0a79e044ed02e9e40e8f15d2c88b5c615dbb 100644
--- a/cc/trees/quad_culler_unittest.cc
+++ b/cc/trees/quad_culler_unittest.cc
@@ -108,7 +108,7 @@ class QuadCullerTest : public testing::Test {
                    TiledLayerImpl* layer,
                    LayerIteratorType* it,
                    OcclusionTrackerImpl* occlusion_tracker) {
-    occlusion_tracker->EnterLayer(*it, false);
+    occlusion_tracker->EnterLayer(*it);
     QuadCuller quad_culler(
         quad_list, shared_state_list, layer, *occlusion_tracker, false, false);
     AppendQuadsData data;
@@ -819,7 +819,7 @@ TEST_F(QuadCullerTest, PartialCullingNotDestroyed) {
   replica_quad->visible_rect = gfx::Rect(30, 40, 15, 16);
 
   // Nothing is occluding.
-  occlusion_tracker.EnterLayer(it, false);
+  occlusion_tracker.EnterLayer(it);
 
   EXPECT_EQ(0u, quad_list.size());
 
@@ -893,7 +893,7 @@ TEST_F(QuadCullerTest, PartialCullingWithOcclusionNotDestroyed) {
   replica_quad->visible_rect = gfx::Rect(10, 30, 15, 16);
 
   // Occlude the left part of the visible rects.
-  occlusion_tracker.EnterLayer(it, false);
+  occlusion_tracker.EnterLayer(it);
   occlusion_tracker.set_occlusion_from_outside_target(gfx::Rect(0, 0, 15, 100));
 
   EXPECT_EQ(0u, quad_list.size());