cc: Block drawing of tiles without twins.

cc/layers/picture_layer_impl.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/layers/picture_layer_impl.h"
 
 #include <algorithm>
 #include <limits>
 
 #include "base/debug/trace_event_argument.h"
@@ skipping 849 matching lines @@
     // active layer is also missing when both this layer and its twin have
     // "simple" sets of tilings: only 2 tilings (high and low) or only 1 high
     // res tiling. This avoids having to iterate/track coverage of non-ideal
     // tilings during the last draw call on the active layer.
     if (tilings_->num_tilings() <= 2 &&
         twin_layer_->tilings_->num_tilings() <= tilings_->num_tilings()) {
       twin_low_res = low_res ? GetTwinTiling(low_res) : NULL;
       twin_high_res = high_res ? GetTwinTiling(high_res) : NULL;
     }
 
+    // When there is a race between activation+drawing and rasterizing,
+    // for tiles without twins, we should prevent the active tree from
+    // drawing these tiles.
+    if (twin_high_res && !twin_high_res->tiles_size()) {
+      layer_tree_impl()->SetRequiresHighResToDraw();

[sohanjg, 2014/09/08 13:45:16]

we should SetRequiresHighResToDraw, on the twin layer impl tree i think, but
that causes couple of tests *.NothingRequiredIfActiveMissingTiles fails, i am
checking them.

Also, will this be right place to block draw ?

+    }
+
     // If this layer and its twin have different transforms, then don't compare
     // them and only allow activating to high res tiles, since tiles on each
     // layer will be in different places on screen.
     if (twin_layer_->layer_tree_impl()->RequiresHighResToDraw() ||
         bounds() != twin_layer_->bounds() ||
         draw_properties().screen_space_transform !=
             twin_layer_->draw_properties().screen_space_transform) {
       twin_high_res = NULL;
       twin_low_res = NULL;
     }
@@ skipping 871 matching lines @@
       PictureLayerTilingSet::TilingRange tiling_range = CurrentTilingRange();
       size_t current_tiling_range_offset = current_tiling_ - tiling_range.start;
       return tiling_range.end - 1 - current_tiling_range_offset;
     }
   }
   NOTREACHED();
   return 0;
 }
 
 }  // namespace cc