InnoDB buffer pool active pages
You can kind of do this from MySQL 5.6 and onwards using information_schema.  I have an example of finding the ideal buffer pool size on my blog here: http://www.tocker.ca/2013/05/31/estimating-mysqls-working-set-with-information_schema.html

The caveat is that you may need to either restart or lower the buffer pool size first.  Inactive pages will just stay in memory if there is no need to make free space - which could skew your result on a server that's been running for a while, yet has plenty of memory.