Aligns behavior with other allocate calls in factory that allow
choosing the generation depending on the use case.

Bug: v8:6771
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I63b95de7e664a51af8ca24a75f2122dfe1792c42
Reviewed-on: https://chromium-review.googlesource.com/642799Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47707}