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

cc/layers/picture_layer_impl.cc

Index: cc/layers/picture_layer_impl.cc
diff --git a/cc/layers/picture_layer_impl.cc b/cc/layers/picture_layer_impl.cc
index 6d8914033e1d43802052d9f9302bb656a1b9e657..fe58d2a618996b479f3875eeea91d85dd4441f85 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -1567,6 +1567,7 @@ void PictureLayerImpl::LayerRasterTileIterator::AdvanceToNextStage() {
 PictureLayerImpl::LayerEvictionTileIterator::LayerEvictionTileIterator()
     : layer_(nullptr),
       tree_priority_(SAME_PRIORITY_FOR_BOTH_TREES),
+      layer_has_twin_layer_(false),
       current_category_(PictureLayerTiling::EVENTUALLY),
       current_tiling_range_type_(PictureLayerTilingSet::HIGHER_THAN_HIGH_RES),
       current_tiling_(0u) {
@@ -1577,6 +1578,7 @@ PictureLayerImpl::LayerEvictionTileIterator::LayerEvictionTileIterator(
     TreePriority tree_priority)
     : layer_(layer),
       tree_priority_(tree_priority),
+      layer_has_twin_layer_(!!layer->GetPendingOrActiveTwinLayer()),
       current_category_(PictureLayerTiling::EVENTUALLY),
       current_tiling_range_type_(PictureLayerTilingSet::HIGHER_THAN_HIGH_RES),
       current_tiling_(0u) {
@@ -1590,9 +1592,8 @@ PictureLayerImpl::LayerEvictionTileIterator::LayerEvictionTileIterator(
       break;
 
     current_iterator_ = PictureLayerTiling::TilingEvictionTileIterator(
-        layer_->tilings_->tiling_at(CurrentTilingIndex()),
-        tree_priority,
-        current_category_);
+        layer_->tilings_->tiling_at(CurrentTilingIndex()), tree_priority,
+        current_category_, layer_has_twin_layer_);
   } while (!current_iterator_);
 }
 
@@ -1619,9 +1620,8 @@ operator++() {
       break;
 
     current_iterator_ = PictureLayerTiling::TilingEvictionTileIterator(
-        layer_->tilings_->tiling_at(CurrentTilingIndex()),
-        tree_priority_,
-        current_category_);
+        layer_->tilings_->tiling_at(CurrentTilingIndex()), tree_priority_,
+        current_category_, layer_has_twin_layer_);
   }
   return *this;
 }
@@ -1633,10 +1633,15 @@ PictureLayerImpl::LayerEvictionTileIterator::operator bool() const {
 bool PictureLayerImpl::LayerEvictionTileIterator::AdvanceToNextCategory() {
   switch (current_category_) {
     case PictureLayerTiling::EVENTUALLY:
+      if (!layer_->HasValidTilePriorities() &&
+          layer_->GetTree() == PENDING_TREE)
+        return false;
       current_category_ =
           PictureLayerTiling::EVENTUALLY_AND_REQUIRED_FOR_ACTIVATION;
       return true;
     case PictureLayerTiling::EVENTUALLY_AND_REQUIRED_FOR_ACTIVATION:
+      if (!layer_->HasValidTilePriorities())
+        return false;
       current_category_ = PictureLayerTiling::SOON;
       return true;
     case PictureLayerTiling::SOON: