Adds checks to ensure that AST rewriting only runs for top-level (or eval)
code, that for these cases it runs on the main thread, and that for
non-top-level code it can safely be called off-thread.

BUG=v8:5832

Review-Url: https://codereview.chromium.org/2630563002
Cr-Commit-Position: refs/heads/master@{#42367}