cc: More consistent reasoning about display list memory usage.

cc/playback/display_item.h

Index: cc/playback/display_item.h
diff --git a/cc/playback/display_item.h b/cc/playback/display_item.h
index f76a13c53cf92a7f8ab5ffd4e9fcfbc9c3530d53..d89807dd3ceb9a94ed065ebf16a16de9e3d44902 100644
--- a/cc/playback/display_item.h
+++ b/cc/playback/display_item.h
@@ -21,11 +21,10 @@ class CC_EXPORT DisplayItem {
 
   void SetNew(bool is_suitable_for_gpu_rasterization,
               int approximate_op_count,
-              size_t picture_memory_usage) {
+              size_t external_memory_usage) {
     is_suitable_for_gpu_rasterization_ = is_suitable_for_gpu_rasterization;
     approximate_op_count_ = approximate_op_count;
-    picture_memory_usage_ =
-        picture_memory_usage + sizeof(bool) + sizeof(int) + sizeof(size_t);
+    external_memory_usage_ = external_memory_usage;
   }
 
   virtual void Raster(SkCanvas* canvas,
@@ -37,14 +36,17 @@ class CC_EXPORT DisplayItem {
     return is_suitable_for_gpu_rasterization_;
   }
   int approximate_op_count() const { return approximate_op_count_; }
-  size_t picture_memory_usage() const { return picture_memory_usage_; }
+  size_t external_memory_usage() const { return external_memory_usage_; }
 
  protected:
   DisplayItem();
 
   bool is_suitable_for_gpu_rasterization_;
   int approximate_op_count_;
-  size_t picture_memory_usage_;
+
+  // The size, in bytes, of the memory owned by this display item but not
+  // allocated within it (e.g. held through scoped_ptr or vector).
+  size_t external_memory_usage_;
 };
 
 }  // namespace cc