cc: Replace Region with SimpleEnclosedRegion for occlusion tracking

cc/layers/layer_impl.cc

Index: cc/layers/layer_impl.cc
diff --git a/cc/layers/layer_impl.cc b/cc/layers/layer_impl.cc
index e6e60f5f3ef7cc1e474c2b5c4a2aef192906b3b7..f28d547bf19f23b8e41e43cb81ac69074fe5f353 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -11,6 +11,7 @@
 #include "cc/animation/animation_registrar.h"
 #include "cc/animation/scrollbar_animation_controller.h"
 #include "cc/base/math_util.h"
+#include "cc/base/simple_enclosed_region.h"
 #include "cc/debug/debug_colors.h"
 #include "cc/debug/layer_tree_debug_state.h"
 #include "cc/debug/micro_benchmark_impl.h"
@@ -1147,10 +1148,10 @@ void LayerImpl::SetDoubleSided(bool double_sided) {
   NoteLayerPropertyChangedForSubtree();
 }
 
-Region LayerImpl::VisibleContentOpaqueRegion() const {
+SimpleEnclosedRegion LayerImpl::VisibleContentOpaqueRegion() const {
   if (contents_opaque())
-    return visible_content_rect();
-  return Region();
+    return SimpleEnclosedRegion(visible_content_rect());
+  return SimpleEnclosedRegion();
 }
 
 void LayerImpl::DidBeginTracing() {}