Get rid of two callsites of effect_tree.FindNodeIndexFromOwningLayerId.

BUG=718564
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2876473005
Cr-Commit-Position: refs/heads/master@{#471112}
diff --git a/cc/trees/occlusion_tracker_unittest.cc b/cc/trees/occlusion_tracker_unittest.cc
index 8389954..a0218f64 100644
--- a/cc/trees/occlusion_tracker_unittest.cc
+++ b/cc/trees/occlusion_tracker_unittest.cc
@@ -78,7 +78,8 @@
 
   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);