Findit identified the culprit r581889 with confidence 70.0% in the config "chromium.mac / Mac10.13 Tests (dbg)"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVyvgELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKHAWNocm9taXVtLm1hYy9NYWMxMC4xMyBUZXN0cyAoZGJnKS80NjIxL3dlYmtpdF9sYXlvdXRfdGVzdHMvYUhSMGNDOTBaWE4wY3k5dFpXUnBZUzl0WldScFlTMXpiM1Z5WTJVdmJXVmthV0Z6YjNWeVkyVXRaSFZ5WVhScGIyNHVhSFJ0YkE9PQwLEhNNYXN0ZXJGbGFrZUFuYWx5c2lzGAEM

If the culprit above is wrong, please file a bug using this link and hit submit:
https://bugs.chromium.org/p/chromium/issues/entry?status=Unconfirmed&labels=Pri-1,Test-Findit-Wrong&components=Tools%3ETest%3EFindit%3EFlakiness&summary=%5BFindit%5D%20Flake%20Analyzer%20-%20Wrong%20result%20for%20http/tests/media/media-source/mediasource-duration.html&comment=Link%20to%20Analysis%3A%20https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVyvgELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKHAWNocm9taXVtLm1hYy9NYWMxMC4xMyBUZXN0cyAoZGJnKS80NjIxL3dlYmtpdF9sYXlvdXRfdGVzdHMvYUhSMGNDOTBaWE4wY3k5dFpXUnBZUzl0WldScFlTMXpiM1Z5WTJVdmJXVmthV0Z6YjNWeVkyVXRaSFZ5WVhScGIyNHVhSFJ0YkE9PQwLEhNNYXN0ZXJGbGFrZUFuYWx5c2lzGAEM

Automatically posted by the findit-for-me app (https://goo.gl/Ot9f7N).

Flake, possibly due to a recent change.  If this flake is not due to the associated change, please unassign yourself and the Component so it can be re-triaged.

I had a whitespace, no way it causes a flake. But see  issue 874228   which I filed.

Thanks for reporting! This is a super-low flakiness case, where the previous revision passed 430/430 tries, and the "culprit" passed 429/430 which Findit interpreted as "flaky" (we recently made the thresholds very tight), causing it to file the bug with 70% confidence (which it shouldn't have considering the change in pass rate went from 100% -> 99.8%.

We will be making changes to measuring flakiness and setting confidence scores to use statistical analysis instead so cases like this can avoid spamming.