The MessageLoopBehavior might change over time: Sometimes we want to
wait because wasm background compilation is going on, sometimes we
don't. This makes the semaphore go out of sync with the task queue (we
always notify it when a new task is scheduled, but we only sometimes
wait on it).
Using a condition variable instead of a semaphore avoids this problem.

R=ahaas@chromium.org

Change-Id: Ib9850efc634f5988d3f824895b6566bd76475985
Reviewed-on: https://chromium-review.googlesource.com/969122Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52057}