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

cc/trees/occlusion_tracker.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/occlusion_tracker.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 332 matching lines @@
                               &stack_.back().occlusion_from_outside_target);
 }
 
 void OcclusionTracker::MarkOccludedBehindLayer(const LayerImpl* layer) {
   DCHECK(!stack_.empty());
   DCHECK_EQ(layer->render_target(), stack_.back().target);
 
   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;
 
   SimpleEnclosedRegion opaque_layer_region = layer->VisibleOpaqueRegion();
   if (opaque_layer_region.IsEmpty())
     return;
 
   DCHECK(layer->visible_layer_rect().Contains(opaque_layer_region.bounds()));
 
@@ skipping 27 matching lines @@
   DCHECK(layer_tree->RootRenderSurface() == stack_.back().target);
   const SimpleEnclosedRegion& occluded =
       stack_.back().occlusion_from_inside_target;
   Region visible_region(screen_space_clip_rect_);
   for (size_t i = 0; i < occluded.GetRegionComplexity(); ++i)
     visible_region.Subtract(occluded.GetRect(i));
   return visible_region;
 }
 
 }  // namespace cc