Adds a comment to GridLayout to explain what DCHECK does

ui/views/layout/grid_layout.cc

Index: ui/views/layout/grid_layout.cc
diff --git a/ui/views/layout/grid_layout.cc b/ui/views/layout/grid_layout.cc
index 2927b88da9d5536e3d2195b58d6bcd9a5bca51af..0e4d4150020a17487f7978a0a633ace5d91a74db 100644
--- a/ui/views/layout/grid_layout.cc
+++ b/ui/views/layout/grid_layout.cc
@@ -995,6 +995,9 @@ void GridLayout::AddViewState(ViewState* view_state) {
 void GridLayout::AddRow(Row* row) {
   current_row_++;
   remaining_row_span_--;
+  // GridLayout requires that if you add a View with a row span you use the same
+  // column set for each of the rows the view lands it. This DCHECK verifies
+  // that.
   DCHECK(remaining_row_span_ <= 0 ||
          row->column_set() == NULL ||
          row->column_set() == GetLastValidColumnSet());