Bug 249024

During the initial steps of masonry, we iterate over all the grid items and separate them into different structures to use in different parts of the algorithm. There are 2 main things that can be done to improve the quality of the code:

1.) Instead of storing the items using raw pointers, we should use WeakPtrs/CheckPtrs or std::reference_wrapper 
2.) At the end of the layout we should probably clear these collections so it is obvious we intend not to use them after layout