Adds new API function to request code cache. Earlier code cache was
produced along with compile requests. This new API allows us to request
code cache after executing. Also adds support in the code serializer to
serialize after executing the script.

Bug: chromium:783124
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Id7b972a2b4c8dcf7a6d9f5ea210890ae968320bd
Reviewed-on: https://chromium-review.googlesource.com/781767Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49717}