This moves the logging of the RCS event for background parsing tasks out
of the parser and performs it at the end of the background parsing task.
This is necessary in order to log background compile RCS events which happen
after parsing.

BUG=v8:5203

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ie216eeade0279d8243818a8eb59309969775823c
Reviewed-on: https://chromium-review.googlesource.com/776669Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49483}