stop using SkXfermode -- use SkBlendMode instead

cc/trees/occlusion_tracker.cc

Index: cc/trees/occlusion_tracker.cc
diff --git a/cc/trees/occlusion_tracker.cc b/cc/trees/occlusion_tracker.cc
index 5ca0f56fe807308636ed7098f33c008c55c8018c..56d237c6665e75a54555018b6f1e91468f731f73 100644
--- a/cc/trees/occlusion_tracker.cc
+++ b/cc/trees/occlusion_tracker.cc
@@ -328,7 +328,7 @@ void OcclusionTracker::MarkOccludedBehindLayer(const LayerImpl* layer) {
 
   // The only currently supported draw_blend_mode is SrcOver mode, so
   // draw_blend_mode does not affect occlusion.
-  DCHECK_EQ(layer->draw_blend_mode(), SkXfermode::kSrcOver_Mode);
+  DCHECK_EQ(layer->draw_blend_mode(), SkBlendMode::kSrcOver);
 
   if (layer->Is3dSorted())
     return;