Enable web_tests editing/selection/offset-from-point-complex-scripts.html on Mac

This patch enables offset-from-point-complex-scripts.html on Mac because
the test is no longer failed on these days[1].

[1] https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=blink_web_tests&showExpectations=true&tests=editing%2Fselection%2Foffset-from-point-complex-scripts.html

Bug: 864888
Change-Id: Iecee744df60dc3956e97d73ba94d1ccbb30e3830
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2497743
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821228}

third_party/blink/web_tests/TestExpectations

@@ -4839,8 +4839,6 @@ crbug.com/862643 http/tests/serviceworker/navigation_preload/use-counter.html [
 # Sheriff 2018-7-13
 crbug.com/863599 [ Debug ] external/wpt/requestidlecallback/callback-iframe.html [ Pass Timeout ]
 
-crbug.com/864888 [ Mac ] editing/selection/offset-from-point-complex-scripts.html [ Failure Pass ]
-
 # Sheriff 2018-07-26
 crbug.com/867982 [ Debug ] external/wpt/requestidlecallback/basic.html [ Timeout Pass ]
 crbug.com/882756 [ Debug ] external/wpt/requestidlecallback/cancel-invoked.html [ Timeout Pass ]