Worker: Remove locking from WorkerThread::postTask()

WorkerThread::postTask() can be called from various places and locking in the
function may lead to deadlock. To avoid the problem, this CL removes locking
from WorkerThread::postTask() and makes it possible to safely access to shared
flags without locking.

BUG=620442

Review-Url: https://codereview.chromium.org/2142273004
Cr-Commit-Position: refs/heads/master@{#405715}
2 files changed