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,chromium:789694
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Id4e6a967e176e3e979dc4ccb9a37a353c70c3890
Reviewed-on: https://chromium-review.googlesource.com/797036Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49793}