[TaskScheduler]: Spawn workers from GetWork().

In an effort to avoid creating threads from the main thread,
this CL avoids the creation of a new idle thread after waking up
the current idle thread from the main thread.
This is done by moving a call to MaintainAtLeastOneIdleWorkerLockRequired()
from WakeUpOneWorkerLockRequired() to GetWork().
Note that a call to MaintainAtLeastOneIdleWorkerLockRequired() is still
done before waking up a worker in WakeUpOneWorkerLockRequired(), in case no
idle thread was created yet.

Bug: 847501
Change-Id: Ib3903c117a781a72f9d6a6aaeceee6554e5b9803
Reviewed-on: https://chromium-review.googlesource.com/c/1338382
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609848}

base/task/task_scheduler/scheduler_worker_pool_impl.cc

@@ -482,6 +482,9 @@ SchedulerWorkerPoolImpl::SchedulerWorkerDelegateImpl::GetWork(
       return nullptr;
     }
 
+    // Replace this worker if it was the last one, capacity permitting.
+    outer_->MaintainAtLeastOneIdleWorkerLockRequired();
+
     // Excess workers should not get work, until they are no longer excess (i.e.
     // max tasks increases or another worker cleans up). This ensures that if we
     // have excess workers in the pool, they get a chance to no longer be excess
@@ -844,10 +847,6 @@ bool SchedulerWorkerPoolImpl::WakeUpOneWorkerLockRequired() {
     }
   }
 
-  // Ensure that there is one worker that can run tasks on top of the idle
-  // stack, capacity permitting.
-  MaintainAtLeastOneIdleWorkerLockRequired();
-
   return true;
 }