In inspector-task.cc, the frontend and backend runners are explicitly
joined before going out-of-scope. On POSIX platforms, calling
pthread_join() on a thread that has already been joined has undefined
behavior. For example, under the musl C runtime library, a successful
call to pthread_join() will unmap the pthread_t thread information
region, and calling pthread_join() again will result in SIGSEGV.

R=clemensb@chromium.org, szuend@chromium.org

Change-Id: Ifdf34ed190df4c722c135ef043a3df588973b984
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2785905Reviewed-by: Simon Zünd <szuend@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73699}