fuchsia: Disable a couple flaky ui_base_unittests

TBR=wez@chromium.org

Bug: 752220, 738275
Change-Id: I839aca92ea3d4421139ce0b2113983bce780dd16
Reviewed-on: https://chromium-review.googlesource.com/600493Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491812}

testing/buildbot/filters/fuchsia.ui_base_unittests.filter

@@ -4,3 +4,7 @@
 # crbug.com/740608 .
 -*DataPackTest.*
 -ResourceBundleImageTest.*
+
+# Flaking, see https://crbug.com/752220.
+-ClipboardTest/0.ClearTest
+-L10nUtilTest.TimeDurationFormatAllLocales