Unskip two idlharness tests

There was a short period of time after we rolled in the new version of
WPT tools (r588057) when these two tests were missing baselines (likely
due to race between test changes and the roll). Now that the baselinse
have been created by the importer, the two tests should no longer fail.

Bug: 879728
Change-Id: Ide735b5dcf3538642187b0646c5459bb0442b03b
Reviewed-on: https://chromium-review.googlesource.com/1214106
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589780}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5134,7 +5134,3 @@ crbug.com/878315 [ Mac ] virtual/threaded/fast/scroll-behavior/overscroll-behavi
 
 # Sheriff 2018-08-23
 crbug.com/877183 [ Linux Win ] external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-intrinsic-ratio-003v.html [ Pass Failure ]
-
-# Sheriff 2018-08-31
-crbug.com/879728 external/wpt/payment-handler/idlharness.https.any.serviceworker.html [ Pass Failure ]
-crbug.com/879728 external/wpt/push-api/idlharness.https.any.serviceworker.html [ Pass Failure ]