Update expectations of longtask-timing

The tests were disabled when determined to be flaky (see CLs in the bug below).
The longtask-in-sibling-iframe.html test seems to pass all the time, and the
other is marked as Pass Timeout instead of Skip.

Bug: 796668
Change-Id: I4d297a0ad1dd5bfac6a6f5d1e0eb0aeb03b65935
Reviewed-on: https://chromium-review.googlesource.com/1135913Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574917}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3461,8 +3461,8 @@ crbug.com/792437 external/wpt/css/css-multicol/multicol-rule-outset-000.xht [ Fa
 crbug.com/636055 external/wpt/css/css-multicol/multicol-span-all-margin-nested-002.xht [ Failure ]
 crbug.com/792446 external/wpt/css/css-multicol/multicol-span-float-001.xht [ Failure ]
 
-crbug.com/796668 external/wpt/longtask-timing/longtask-in-sibling-iframe.html [ Skip ]
-crbug.com/796668 external/wpt/longtask-timing/longtask-in-sibling-iframe-crossorigin.html [ Skip ]
+crbug.com/796668 external/wpt/longtask-timing/longtask-in-sibling-iframe.html [ Pass Timeout ]
+crbug.com/796668 external/wpt/longtask-timing/longtask-in-sibling-iframe-crossorigin.html [ Pass Timeout ]
 
 # This test times out on debug builds, see https://crbug.com/755810
 crbug.com/626703 [ Debug ] external/wpt/html/semantics/tabular-data/processing-model-1/span-limits.html [ Skip ]