This fixes a deadlock related to throttling: It can happen that all
background tasks detect that they should not produce more work because
of throttling (!CanAcceptWork()). Reducing the number of running
background tasks is done in a later step (OnBackgroundTaskStopped).
If the finisher task finishes all outstanding units between these two
calls, it will not schedule another background compilation task, but
all background compilation tasks will quit, hence compilation will
never finish.

Fixing this should allow us to reenable the 'wasm-finish-compilation'
test: https://crrev.com/c/999632

R=ahaas@chromium.org

Bug: chromium:824681
Change-Id: I967e4d6b2917d369dd49bb80ce4bef552d10b371
Reviewed-on: https://chromium-review.googlesource.com/1002174
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52483}