Memory profile tables should scroll together

[elliette]

When a developer enables "Advanced developer settings" from the general DevTools setting dialog, the Memory panel includes a table that shows GC statistics.

This table is above the memory profile table, and looks like it is part of the memory profile table. However, the two tables are separate, and scroll separately.

We should figure out a way to get them to scroll horizontally together, possibly by adding a MultiTable widget that supports multiple tables.