cc: early return if unoccluded_region_in_target_surface is empty

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 <algorithm>
 
 #include "cc/base/math_util.h"
 #include "cc/layers/layer.h"
@@ skipping 549 matching lines @@
 
   // Take the ToEnclosingRect at each step, as we want to contain any unoccluded
   // partial pixels in the resulting Rect.
   Region unoccluded_region_in_target_surface =
       MathUtil::MapEnclosingClippedRect(draw_transform, content_rect);
   unoccluded_region_in_target_surface.Subtract(
       stack_.back().occlusion_from_inside_target);
   unoccluded_region_in_target_surface.Subtract(
       stack_.back().occlusion_from_outside_target);
 
+  if (unoccluded_region_in_target_surface.IsEmpty())
+    return gfx::Rect();
+
   gfx::Rect unoccluded_rect_in_target_surface =
       unoccluded_region_in_target_surface.bounds();
   gfx::Rect unoccluded_rect = MathUtil::ProjectEnclosingClippedRect(
       inverse_draw_transform, unoccluded_rect_in_target_surface);
   unoccluded_rect.Intersect(content_rect);
 
   return unoccluded_rect;
 }
 
 template <typename LayerType>
@@ skipping 22 matching lines @@
 
   // Take the ToEnclosingRect at each step, as we want to contain any unoccluded
   // partial pixels in the resulting Rect.
   Region unoccluded_region_in_target_surface =
       MathUtil::MapEnclosingClippedRect(draw_transform, content_rect);
   unoccluded_region_in_target_surface.Subtract(
       second_last.occlusion_from_inside_target);
   unoccluded_region_in_target_surface.Subtract(
       second_last.occlusion_from_outside_target);
 
+  if (unoccluded_region_in_target_surface.IsEmpty())
+    return gfx::Rect();
+
   gfx::Rect unoccluded_rect_in_target_surface =
       unoccluded_region_in_target_surface.bounds();
   gfx::Rect unoccluded_rect = MathUtil::ProjectEnclosingClippedRect(
       inverse_draw_transform, unoccluded_rect_in_target_surface);
   unoccluded_rect.Intersect(content_rect);
 
   return unoccluded_rect;
 }
 
 // Instantiate (and export) templates here for the linker.
 template class OcclusionTracker<Layer>;
 template class OcclusionTracker<LayerImpl>;
 
 }  // namespace cc