We don't need a lock for the output queue on the main thread

We don't need a lock for the output queue on the main thread

The output queue is supposed to be a lock-free anyways, we're just
temporarily abusing it by having multiple producers. For those, we need
the lock when enqueuing jobs.

BUG=none
R=yangguo@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/953883002

Cr-Commit-Position: refs/heads/master@{#26824}