fuchsia: Re-enable BuildTime.InThePast, works on arm64 now

Bug: 779668
Change-Id: If23ef86c79f73581b3330f8fa5d020212f74c518
Reviewed-on: https://chromium-review.googlesource.com/747201
Commit-Queue: Scott Graham <scottmg@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512879}

testing/buildbot/filters/fuchsia.base_unittests.filter

@@ -62,6 +62,3 @@
 # https://crbug.com/779645 - requires ability to set address-space limits.
 -PartitionAllocTest.RepeatedReturnNull
 -PartitionAllocTest.RepeatedReturnNullDirect
-
-# https://crbug.com/779668 - fails under Fuchsia/ARM64 due to Time::Now issue.
--BuildTime.InThePast