cc: Speed up CleanUpImageDecodeTasks a bit.

cc/resources/tile_manager.h

Index: cc/resources/tile_manager.h
diff --git a/cc/resources/tile_manager.h b/cc/resources/tile_manager.h
index 1e5d7ff1cd5dbce91bfaceabc961b4a1500bcb66..2fc145675ec59a4e8c224b39a0926d0ab38eb52e 100644
--- a/cc/resources/tile_manager.h
+++ b/cc/resources/tile_manager.h
@@ -176,6 +176,9 @@ class CC_EXPORT TileManager : public RasterWorkerPoolClient {
   typedef base::hash_map<int, PixelRefTaskMap> LayerPixelRefTaskMap;
   LayerPixelRefTaskMap image_decode_tasks_;
 
+  typedef base::hash_map<int, int> LayerCountMap;

[tomhudson, 2013/09/04 18:38:34]

I assume we're using hash_map here instead of something directly addressable
because the range of layer IDs is large and very sparsely used?

[enne (OOO), 2013/09/04 18:45:24]

It really depends on the page.  Scrollbars in particular get attached and
detached a lot, burning through layer id space.  I think it would be unwise to
assume that layer ids are tightly packed or small in range.

[vmpstr, 2013/09/04 19:36:40]

Right.. I think it would be a waste to grow this to the max of all layer ids (in
order to be directly addressable. From the few pages I looked at, it seems that
we have a handful of layers, but their IDs can be in the range of >1000 (for
instance on sky.com)

+  LayerCountMap used_layer_counts_;
+
   RasterTaskCompletionStats update_visible_tiles_stats_;
 
   DISALLOW_COPY_AND_ASSIGN(TileManager);