[WIP] cc: Use reverse spiral iterator in tiling eviction.

cc/resources/eviction_tile_priority_queue.cc

Index: cc/resources/eviction_tile_priority_queue.cc
diff --git a/cc/resources/eviction_tile_priority_queue.cc b/cc/resources/eviction_tile_priority_queue.cc
index 9f05e816764613ee7424cff8bff3d6f6376e8430..00c58e63cbd7c37baebdd8686cc244705084677c 100644
--- a/cc/resources/eviction_tile_priority_queue.cc
+++ b/cc/resources/eviction_tile_priority_queue.cc
@@ -157,9 +157,7 @@ Tile* EvictionTilePriorityQueue::PairedPictureLayerQueue::Top(
   DCHECK(*next_iterator);
 
   Tile* tile = **next_iterator;
-  DCHECK(std::find(returned_shared_tiles.begin(),
-                   returned_shared_tiles.end(),
-                   tile) == returned_shared_tiles.end());
+  DCHECK(returned_tiles_for_debug.find(tile) == returned_tiles_for_debug.end());
   return tile;
 }
 
@@ -171,25 +169,11 @@ void EvictionTilePriorityQueue::PairedPictureLayerQueue::Pop(
   PictureLayerImpl::LayerEvictionTileIterator* next_iterator =
       next_tree == ACTIVE_TREE ? &active_iterator : &pending_iterator;
   DCHECK(*next_iterator);
-  returned_shared_tiles.push_back(**next_iterator);
+  DCHECK(returned_tiles_for_debug.insert(**next_iterator).second);
   ++(*next_iterator);
 
-  if (IsEmpty())
-    return;
-
-  next_tree = NextTileIteratorTree(tree_priority);
-  next_iterator =
-      next_tree == ACTIVE_TREE ? &active_iterator : &pending_iterator;
-  while (std::find(returned_shared_tiles.begin(),
-                   returned_shared_tiles.end(),
-                   **next_iterator) != returned_shared_tiles.end()) {
-    ++(*next_iterator);
-    if (IsEmpty())
-      break;
-    next_tree = NextTileIteratorTree(tree_priority);
-    next_iterator =
-        next_tree == ACTIVE_TREE ? &active_iterator : &pending_iterator;
-  }
+  // If no empty, use Top to do DCHECK the next iterator.
+  DCHECK(IsEmpty() || Top(tree_priority));
 }
 
 WhichTree
@@ -213,6 +197,15 @@ EvictionTilePriorityQueue::PairedPictureLayerQueue::NextTileIteratorTree(
   const TilePriority& pending_priority =
       pending_tile->priority_for_tree_priority(tree_priority);
 
+  if (active_priority.priority_bin == pending_priority.priority_bin) {
+    if (active_tile->required_for_activation()) {
+      if (!pending_tile->required_for_activation())
+        return PENDING_TREE;
+    } else if (pending_tile->required_for_activation()) {
+      return ACTIVE_TREE;
+    }
+  }
+
   if (pending_priority.IsHigherPriorityThan(active_priority))
     return ACTIVE_TREE;
   return PENDING_TREE;