[Fuchsia+TaskScheduler] Re-enable tests that were flaky because of rogue SchedulerWorkers.

Rogue SchedulerWorkers were fixed once and for all in crbug.com/827615

R=wez@chromium.org

Bug: 816170, 827615
Change-Id: I697124ed7cd5282bce8e504427e1daa6adc08df2
Reviewed-on: https://chromium-review.googlesource.com/1031210
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554386}

testing/buildbot/filters/fuchsia.base_unittests.filter

@@ -76,11 +76,5 @@
 # Flaky, https://crbug.com/810077.
 -MessageLoopTypedTest.RecursivePosts/*
 
-# Flaky, https://crbug.com/735701 and https://crbug.com/816170.
--TaskSchedulerWorkerPoolHistogramTest.NumTasks*Cleanup
-
 # Flaky, https://crbug.com/811881
 -AddressSpaceRandomizationTest.RandomBitCorrelations*
-
-# Flaky, https://crbug.com/813651
--TaskSchedulerWorkerPoolCheckTlsReuse.CheckCleanupWorkers