mix-blend-mode implementation for accelerated layers

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 2bd9b552e8bb856fcc1434fdf262e54a4ad511e0..52e76c9c5322549e2de000f025bd5670c2e9007d 100644
--- a/cc/trees/occlusion_tracker.cc
+++ b/cc/trees/occlusion_tracker.cc
@@ -233,9 +233,9 @@ void OcclusionTrackerBase<LayerType, RenderSurfaceType>::FinishedRenderTarget(
 
   // If the occlusion within the surface can not be applied to things outside of
   // the surface's subtree, then clear the occlusion here so it won't be used.
-  if (finished_target->mask_layer() ||
-      !SurfaceOpacityKnown(surface) ||
+  if (finished_target->mask_layer() || !SurfaceOpacityKnown(surface) ||
       surface->draw_opacity() < 1 ||
+      !finished_target->uses_default_blend_mode() ||
       target_is_only_for_copy_request ||
       finished_target->filters().HasFilterThatAffectsOpacity()) {
     stack_.back().occlusion_from_outside_target.Clear();
@@ -420,6 +420,9 @@ void OcclusionTrackerBase<LayerType, RenderSurfaceType>::
   if (!LayerOpacityKnown(layer) || layer->draw_opacity() < 1)
     return;
 
+  if (!layer->uses_default_blend_mode())
+    return;
+
   if (LayerIsInUnsorted3dRenderingContext(layer))
     return;