cc: Ensure that skewport.Contains(visible_rect) is always true.

cc/tiles/picture_layer_tiling.cc

 // Copyright 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 "cc/tiles/picture_layer_tiling.h"
 
 #include <algorithm>
 #include <cmath>
 #include <limits>
 #include <set>
@@ skipping 534 matching lines @@
                  extrapolation_multiplier * (old_right - new_right),
                  extrapolation_multiplier * (old_bottom - new_bottom));
 
   // Ensure that visible rect is contained in the skewport.
   skewport.Union(visible_rect_in_content_space);
 
   // Clip the skewport to |max_skewport|. This needs to happen after the
   // union in case intersecting would have left the empty rect.
   skewport.Intersect(max_skewport);
 
+  // Due to limits in int's representation, it is possible that the two
+  // operations above (union and intersect) result in an empty skewport. To
+  // avoid any unpleasant situations like that, union the visible rect again to
+  // ensure that skewport.Contains(visible_rect_in_content_space) is always
+  // true.
+  skewport.Union(visible_rect_in_content_space);
+
   return skewport;
 }
 
 bool PictureLayerTiling::ComputeTilePriorityRects(
     const gfx::Rect& viewport_in_layer_space,
     float ideal_contents_scale,
     double current_frame_time_in_seconds,
     const Occlusion& occlusion_in_layer_space) {
   if (!NeedsUpdateForFrameAtTimeAndViewport(current_frame_time_in_seconds,
                                             viewport_in_layer_space)) {
@@ skipping 506 matching lines @@
       break;
   }
 
   gfx::Rect result(origin_x, origin_y, width, height);
   if (cache)
     cache->previous_result = result;
   return result;
 }
 
 }  // namespace cc