jam@ pointed out that no tests are hitting the OnBeginNavigation logic ananta@ added in https://codereview.chromium.org/2701093002/.  However, as I determined in https://codereview.chromium.org/2890613002/#msg15, that code is critical to preventing a regression of  issue 657896  in some race outcomes.

We should try to add a content_browsertest that covers this branch of the race, where the new JS navigation arrives in OnBeginNavigation before the history navigation gets to PlzNavigate's CommitNavigation stage.