Base: Fix a deadlock during shutdown of SequencedWorkerPool.

During shutdown, the SequencedWorkerPool was destroying some tasks without
releasing its global lock. Any task whose destruction implicitly triggered a
lock access would deadlock.

BUG=448910

Review URL: https://codereview.chromium.org/956583004

Cr-Commit-Position: refs/heads/master@{#318283}
2 files changed