We either want to add code+literals to the map, or just literals.
A recent change in the structure of the map (it now uses WeakCells)
meant that we have to be more clear about what we want to do the right
thing.

BUG=

Review URL: https://codereview.chromium.org/1516833002

Cr-Commit-Position: refs/heads/master@{#32761}