There was a small race where an idle task could be posted after the compiler dispatcher
had aborted and the CancellableTaskRunner had been cancelled. This was causing flakyness
on the bots. This fixes this by moving the idle task posting into the same lock block as
the notification to the main thread that the background task has completed.

BUG=v8:8041

Change-Id: I43ca4cea807bfdfeb13f6d1c4a67a4d8a4f6291f
Reviewed-on: https://chromium-review.googlesource.com/c/1278494Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56607}