cc: Fix DCHECK when impl thread changes scale on a layer being hidden.

cc/resources/picture_layer_tiling_set.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/resources/picture_layer_tiling_set.h"
 
 #include <limits>
 #include <set>
 #include <vector>
 
@@ skipping 47 matching lines @@
     const PictureLayerTilingSet* twin_set,
     const Region& layer_invalidation,
     float minimum_contents_scale,
     float maximum_contents_scale) {
   RemoveTilingsBelowScale(minimum_contents_scale);
   RemoveTilingsAboveScale(maximum_contents_scale);
 
   // Copy over tilings that are shared with the |twin_set| tiling set (if it
   // exists).
   if (twin_set) {
+    if (twin_set->tilings_.empty()) {
+      // If the twin (pending) tiling set is empty, it was not updated for the
+      // current frame. So we drop tilings from our set as well, instead of
+      // leaving behind unshared tilings that are all non-ideal.
+      RemoveAllTilings();
+    }
+
     for (PictureLayerTiling* twin_tiling : twin_set->tilings_) {
       float contents_scale = twin_tiling->contents_scale();
       DCHECK_GE(contents_scale, minimum_contents_scale);
       DCHECK_LE(contents_scale, maximum_contents_scale);
 
       PictureLayerTiling* this_tiling = FindTilingWithScale(contents_scale);
       if (!this_tiling) {
         scoped_ptr<PictureLayerTiling> new_tiling = PictureLayerTiling::Create(
             contents_scale, raster_source, client_,
             max_tiles_for_interest_area_, skewport_target_time_in_seconds_,
@@ skipping 458 matching lines @@
     case LOWER_THAN_LOW_RES:
       range = TilingRange(low_res_range.end, tilings_.size());
       break;
   }
 
   DCHECK_LE(range.start, range.end);
   return range;
 }
 
 }  // namespace cc