Now that SharedFunctionInfos have a unique ID (and the IDs are dense),
we can use them as an index into an array, instead of using a
WeakFixedArray where we have to do a linear scan.

Hooking up liveedit is a bit more involved, see
https://docs.google.com/presentation/d/1FtNa3U7WsF5bPhY9uGoJG5Y9hnz5VBDabfOWpb4unWI/edit
for an overview

BUG=v8:5589
R=verwaest@chromium.org,jgruber@chromium.org

Review-Url: https://codereview.chromium.org/2547483002
Cr-Commit-Position: refs/heads/master@{#41600}