This is in preparation for running code generation off the main thread.

Bug: v8:6048
Change-Id: Ic1ddcbbb8995d49dfcef1ffbf135117291a00ea7
Reviewed-on: https://chromium-review.googlesource.com/505513Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45334}