Add underlays and split off common overlay functionality

cc/quads/list_container.h

Index: cc/quads/list_container.h
diff --git a/cc/quads/list_container.h b/cc/quads/list_container.h
index dd313fc8b45bc67d1b7568bea2dd0d0fe81e6a14..94754aba81aab855041776c761cdb1d1dc0adb73 100644
--- a/cc/quads/list_container.h
+++ b/cc/quads/list_container.h
@@ -209,6 +209,12 @@ class CC_EXPORT ListContainer {
     return new (Allocate(sizeof(DerivedElementType)))
         DerivedElementType(*source);
   }
+  // Construct a new element on top of an existing one.
+  template <typename DerivedElementType>
+  DerivedElementType* ReplaceExistingElement(Iterator at) {
+    at->~BaseElementType();
+    return new (*at) DerivedElementType();
+  }
 
   size_t size() const;
   bool empty() const;