Revert "Revert "SharedMemory: break Cleaner reference cycle.""

This commit was reverted in Q because it broke things, but we want it in
master.

This reverts commit c36d0765a25d4701980738dc3e2053f19eb3d6b8.

Change-Id: I809d9191eee4909d265d2864ebd523f262f6bb61
Test: treehugger
1 file changed