cc: Fix occlusion tracking for layers with non-default blend modes

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 3b69c68318815065a5840b54e632fe6931ac250b..ad2f1a8460dc729a4e5a9a1fd22b5c72af923afe 100644
--- a/cc/trees/occlusion_tracker.cc
+++ b/cc/trees/occlusion_tracker.cc
@@ -350,8 +350,9 @@ void OcclusionTracker::MarkOccludedBehindLayer(const LayerImpl* layer) {
   if (layer->draw_opacity() < 1)
     return;
 
-  if (!layer->uses_default_blend_mode())
-    return;
+  // 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);

[ajuma, 2016/07/25 18:34:42]

An alternative would be to delete draw_blend_mode for now. There's an open bug
for not always creating surfaces for blend mode (http://crbug.com/301738) but
it's been open for a while and is currently unowned.

 
   if (layer->Is3dSorted())
     return;