fuchsia: Filter out some known-flake tests in base_unittests

Bug: 738275, 735701, 750370, 750756
Change-Id: Ibaa143616ce60ae6d94854f9cadc8daf2d84a37c
Reviewed-on: https://chromium-review.googlesource.com/596914Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491200}

testing/buildbot/filters/fuchsia.base_unittests.filter

@@ -87,3 +87,15 @@
 
 # CancelableSyncSocket tests fail: http://crbug.com/741783
 -CancelableSyncSocket*
+
+# These tests are occasionally flaking. See https://crbug.com/738275. Please be
+# pretty confident you've fixed their rarely-flakiness before re-enabling.
+-AllModes/TaskSchedulerSingleThreadTaskRunnerManagerCommonTest.*
+-AllocationRegisterTest.InsertRemoveRandomOrder
+-MemoryPeakDetectorTest.StdDev
+-MemoryPeakDetectorTest.Throttle
+-ObserverListThreadSafeTest.CrossThreadObserver
+-OneTraitsExecutionModePair/TaskSchedulerImplTest.*
+-ProcessUtilTest.DelayedTermination
+-ProcessUtilTest.ImmediateTermination
+-TraceCategoryTest.ThreadRaces