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Before this CL, logic to wait for a WorkCallback to run or be
abandoned was duplicated in multiple
ModelSafeWorker::DoWorkAndWaitUntilDoneImpl() implementations.

With this CL, ModelSafeWorker::DoWorkAndWaitUntilDoneImpl() is
replaced with ModelSafeWorker::ScheduleWork(). Implementations
are merely responsible for scheduling the callback that they
receive; they don't have to wait for it to run or be abandoned.

The logic to wait for the callback to run or be abandoned is moved
into ModelSafeWorker::DoWorkAndWaitUntilDone().
ModelSafeWorker::DoWorkAndWaitUntilDone() calls
ModelSafeWorker::ScheduleWork(). Then, it waits until either the
callback that it passed to ModelSafeWorker::ScheduleWork() is deleted
(may or may not have run) or ModelSafeWorker::RequestStop() is called
before the callback has started to run. Additionnaly, calling
ModelSafeWorker::RequestStop() transforms into no-op further
invocations of callbacks passed to ModelSafeWorker::ScheduleWork().

Unblocking ModelSafeWorker::DoWorkAndWaitUntilDone() when
ModelSafeWorker::RequestStop() is called is important to
prevent deadlocks in workers that schedule work on the UI
thread. Indeed, in Chrome, no tasks are scheduled on the UI
thread after ModelSafeWorker::RequestStop() is called and
pending UI tasks are not scheduled until after the sync thread
is joined.

BUG=663600

Review-Url: https://codereview.chromium.org/2782573002
Cr-Commit-Position: refs/heads/master@{#463413}