For flushing the input queue, use the input queue lock. Introduce an
explicit refcount to make sure we don't delete the thread object before
all jobs are finished.

BUG=v8:3608
R=yangguo@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/948863002

Cr-Commit-Position: refs/heads/master@{#26812}