Revert "EventTiming: mark event-timing-onloadthenobserve.html as flaky on Mac"

This reverts commit 0850df10.

Reason for revert: the culprit CL was reverted, no longer needed.

Original change's description:
> EventTiming: mark event-timing-onloadthenobserve.html as flaky on Mac
> 
> Bug: 907948
> TBR=tdresser@chromium.org
> 
> Change-Id: I53d328b935ab235e441f555e625aa97fac1b9ea4
> Reviewed-on: https://chromium-review.googlesource.com/c/1349033
> Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
> Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#610519}

TBR=tdresser@chromium.org,npm@chromium.org

Change-Id: Ib20b5fd4bf0cf147518967ede801d9092e7a7112
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 907948
Reviewed-on: https://chromium-review.googlesource.com/c/1349496Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610613}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2793,7 +2793,6 @@ crbug.com/763830 virtual/outofblink-cors-ns/http/tests/security/cors-rfc1918/ [
 
 crbug.com/831729 http/tests/event-timing/event-timing-crossiframe.html [ Timeout ]
 crbug.com/831729 http/tests/event-timing/event-timing-observer-manual.html [ Skip ]
-crbug.com/907949 [ Mac ] http/tests/event-timing/event-timing-onloadthenobserve.html [ Pass Failure ]
 
 # Working on getting the CSP tests going:
 crbug.com/694525 external/wpt/content-security-policy/connect-src/worker-from-guid.sub.html [ Skip ]