Add cheapness predictor success histogram

cc/picture_pile_impl.cc

Index: cc/picture_pile_impl.cc
diff --git a/cc/picture_pile_impl.cc b/cc/picture_pile_impl.cc
index 35300bd70a5b691a5aa1558eed5edf76f3dd095d..f32ed1dc78d026baad81b16226e2e81d464e405d 100644
--- a/cc/picture_pile_impl.cc
+++ b/cc/picture_pile_impl.cc
@@ -168,4 +168,28 @@ skia::RefPtr<SkPicture> PicturePileImpl::GetFlattenedPicture() {
   return picture;
 }
 
+bool PicturePileImpl::IsCheapInRect(
+    gfx::Rect content_rect, float contents_scale) const {
+  gfx::Rect layer_rect = gfx::ToEnclosingRect(
+      gfx::ScaleRect(content_rect, 1.f / contents_scale));
+
+  for (TilingData::Iterator tile_iter(&tiling_, layer_rect);
+       tile_iter; ++tile_iter) {
+    PictureListMap::const_iterator map_iter =
+        picture_list_map_.find(tile_iter.index());
+    if (map_iter == picture_list_map_.end())
+      continue;
+
+    const PictureList& pic_list = map_iter->second;
+    for (PictureList::const_iterator i = pic_list.begin();
+         i != pic_list.end(); ++i) {
+      if (!(*i)->LayerRect().Intersects(layer_rect) || !(*i)->HasRecording())
+        continue;
+      if (!(*i)->IsCheapInRect(layer_rect))

[Tom Hudson, 2013/02/06 12:24:57]

This layer_rect is the content_rect, inverse scaled by contents_scale.

The Picture in the iterator also has a layer_rect_, which often seems to be the
same in practice (and you check to make sure they intersect).

Shouldn't we be worried about the tile bounds somewhere?

+        return false;
+    }
+  }
+  return true;
+}
+
 }  // namespace cc