Get rid of two callsites of effect_tree.FindNodeIndexFromOwningLayerId.

cc/trees/occlusion_tracker_unittest.cc

Index: cc/trees/occlusion_tracker_unittest.cc
diff --git a/cc/trees/occlusion_tracker_unittest.cc b/cc/trees/occlusion_tracker_unittest.cc
index 8389954958e4689b8ff28fc37e6b81cb151a878e..a0218f644974bf64ffe07f71fd2749440ce15b7c 100644
--- a/cc/trees/occlusion_tracker_unittest.cc
+++ b/cc/trees/occlusion_tracker_unittest.cc
@@ -78,7 +78,8 @@ class TestOcclusionTrackerWithClip : public TestOcclusionTracker {
 
   gfx::Rect UnoccludedSurfaceContentRect(const LayerImpl* layer,
                                          const gfx::Rect& content_rect) const {
-    const RenderSurfaceImpl* surface = GetRenderSurface(layer);
+    const RenderSurfaceImpl* surface =
+        GetRenderSurface(const_cast<LayerImpl*>(layer));
     return this->GetCurrentOcclusionForContributingSurface(
                      surface->draw_transform())
         .GetUnoccludedContentRect(content_rect);