cc: Don't require low res to activate for shared ready-to-draw tiles.

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 <set>
 
@@ skipping 918 matching lines @@
     // covered by acceptable tiles at other scales.
     if (!missing_region.Intersects(iter.geometry_rect()))
       continue;
 
     // If the twin tile doesn't exist (i.e. missing recording or so far away
     // that it is outside the visible tile rect) or this tile is shared between
     // with the twin, then this tile isn't required to prevent flashing.
     if (optional_twin_tiling) {
       Tile* twin_tile = optional_twin_tiling->TileAt(iter.i(), iter.j());
       if (!twin_tile || twin_tile == tile) {
-        twin_had_missing_tile = true;
+        // However if the shared tile is being used on the active tree, then
+        // there's no missing content in this place, and low res is not needed.
+        if (!twin_tile || !twin_tile->IsReadyToDraw())
+          twin_had_missing_tile = true;
         continue;
       }
     }
 
     tile->MarkRequiredForActivation();
   }
   return twin_had_missing_tile;
 }
 
 void PictureLayerImpl::DoPostCommitInitialization() {
@@ skipping 819 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