This is a workaround for the fact that %SetCode can "lose" the script for a js native. If the js native is re-initialized (for a Realm or something), then the source SharedFunctionInfo won't have a script anymore. Nonetheless, we may want to optimize the function. If we've compiled bytecode, then we can compile optimized code without a script.

Here, we carve out a special exception for this case, so that we can turn on the --mark-shared-functions-for-tier-up.

BUG=v8:5946
R=leszeks@chromium.org

Review-Url: https://codereview.chromium.org/2684033007
Cr-Commit-Position: refs/heads/master@{#43240}