Chrome Version       : 63.0.3239.132
OS Version: OS X 10.12.6

What steps will reproduce the problem?
1. Open normal tab, for example https://www.chromium.org/
2. Close normal tab.
3. Open tab with Onbeforeunload handler, for example http://www.4guysfromrolla.com/demos/OnBeforeUnloadDemo1.htm
4. Click into tab, because Onbeforeunload handler is not called for page that user didn't interact.
5. Close tab with onbeforeunload handler.
6. Confiration dialog appears, wait about seconds (to see big time delay clearly on histogram), then confirm closing.
7. Observe Tab.Close.UnloadTime histogram on chrome://histograms page.

What is the expected result?
no unusually big values in Tab.Close.UnloadTime samples 

What happens instead of that?
there are unusually big values in Tab.Close.UnloadTime samples

Please provide any additional information below. Attach a screenshot if
possible.

UserAgentString: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 YaBrowser/18.1.1.841 Yowser/2.5 Safari/537.36