Revert of Clear ui::Layer damaged_region_ after commit.

ui/compositor/layer.cc

Index: ui/compositor/layer.cc
diff --git a/ui/compositor/layer.cc b/ui/compositor/layer.cc
index dabbedd73eee8b0fd00f885268d448f95fb36341..e23e87fb63b223517a354fa7034eaab5a434ba61 100644
--- a/ui/compositor/layer.cc
+++ b/ui/compositor/layer.cc
@@ -689,9 +689,9 @@
 
   for (cc::Region::Iterator iter(damaged_region_); iter.has_rect(); iter.next())
     cc_layer_->SetNeedsDisplayRect(iter.rect());
-
-  if (content_layer_)
-    paint_region_.Union(damaged_region_);
+}
+
+void Layer::ClearDamagedRects() {
   damaged_region_.Clear();
 }
 
@@ -756,8 +756,8 @@
   TRACE_EVENT1("ui", "Layer::PaintContentsToDisplayList", "name", name_);
   gfx::Rect local_bounds(bounds().size());
   gfx::Rect invalidation(
-      gfx::IntersectRects(paint_region_.bounds(), local_bounds));
-  paint_region_.Clear();
+      gfx::IntersectRects(damaged_region_.bounds(), local_bounds));
+  ClearDamagedRects();
   cc::DisplayItemListSettings settings;
   settings.use_cached_picture = false;
   scoped_refptr<cc::DisplayItemList> display_list =