Adds a comment to GridLayout to explain what DCHECK does

ui/views/layout/grid_layout.cc

 // Copyright (c) 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 "ui/views/layout/grid_layout.h"
 
 #include <algorithm>
 
 #include "base/logging.h"
 #include "base/stl_util.h"
@@ skipping 977 matching lines @@
                                                     view_states_.end(),
                                                     view_state,
                                                     CompareByRowSpan);
   view_states_.insert(i, view_state);
   SkipPaddingColumns();
 }
 
 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());
   next_column_ = 0;
   rows_.push_back(row);
   current_row_col_set_ = row->column_set();
   SkipPaddingColumns();
 }
 
 void GridLayout::UpdateRemainingHeightFromRows(ViewState* view_state) {
@@ skipping 56 matching lines @@
 
 ColumnSet* GridLayout::GetLastValidColumnSet() {
   for (int i = current_row_ - 1; i >= 0; --i) {
     if (rows_[i]->column_set())
       return rows_[i]->column_set();
   }
   return NULL;
 }
 
 }  // namespace views