Bytecode array visitor has a side-effect of incrementing the age counter.

This patch makes the increment atomic and thus safe for the concurrent
marker.

Bug: chromium:694255
Change-Id: I36c65b02ace8d366206bd8295e72aaa19742ed56
Reviewed-on: https://chromium-review.googlesource.com/610001
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47317}