Adds some additional RCS counters to correctly account background
compilation to the background thread.

Also adds a ParseBackgroundProgram as a top-level event for background
parsing since otherwise only pre-parsing was being tracked.

Perf Sheriffs: Note this is likely to increase the Parse-Background
bucket in v8.runtime_stats benchmarks as it now accounts all background
parsing correclty.

BUG=v8:5203

Change-Id: I6ff614b725d85b0bc1901a7bf0e2bac8de1f7cff
Reviewed-on: https://chromium-review.googlesource.com/786237Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49603}