When disposing the compilation jobs, we have to update some fields
on JSFunction. Though the current implementation is safe given we only
dispose the compilation jobs when main thread is blocked, it is still
cleaner and safer to dispose of these jobs only on the main thread.
That also unifies the way we handle flushing the queues when we want
to block waiting for the pending tasks to finish or not

Basically this cl flushes the input queue before waiting for any
pending tasks to finish. This would avoid the special handling on the
background threads that dispose of these jobs when we are in the
flushing mode. This also means we don't need to keep track of
the mode anymore.

Change-Id: Icd3adbe998612159e796b2bc90486d38c420f9e8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2726502Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73190}