Security feature: pick compiled functions randomly

Bug: v8:6417
Change-Id: Ibcef9ba94e3730a47cd43d2a59b23d5cee3fad40
Reviewed-on: https://chromium-review.googlesource.com/511842
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45472}