Fix for 424682: Draw correct repaint regions instead of a conservative bounding box

cc/debug/debug_rect_history.cc

Index: cc/debug/debug_rect_history.cc
diff --git a/cc/debug/debug_rect_history.cc b/cc/debug/debug_rect_history.cc
index cde7765383f33ba59dccd77278663f9257d3976e..113b90d49c12889a938fc657fdf14ba86e403c08 100644
--- a/cc/debug/debug_rect_history.cc
+++ b/cc/debug/debug_rect_history.cc
@@ -73,12 +73,18 @@ void DebugRectHistory::SavePaintRects(LayerImpl* layer) {
                         static_cast<float>(layer->bounds().width());
     float height_scale = layer->content_bounds().height() /
                          static_cast<float>(layer->bounds().height());
-    gfx::Rect update_content_rect = gfx::ScaleToEnclosingRect(
-        layer->update_rect(), width_scale, height_scale);
-    debug_rects_.push_back(
+
+    for (Region::Iterator it(layer->GetInvalidationRegion());
+        it.has_rect();
+        it.next()) {
+
+      gfx::Rect update_content_rect = gfx::ScaleToEnclosingRect(
+        it.rect(), width_scale, height_scale);
+      debug_rects_.push_back(
         DebugRect(PAINT_RECT_TYPE,
-                  MathUtil::MapEnclosingClippedRect(
-                      layer->screen_space_transform(), update_content_rect)));
+          MathUtil::MapEnclosingClippedRect(
+            layer->screen_space_transform(), update_content_rect)));
+    }
   }
 
   for (unsigned i = 0; i < layer->children().size(); ++i)