Which hero gets a "Game Won" in the statistics if I switch heroes mid-game?

At the end of a match, character win/loss statistics are attributed as a fraction corresponding to the percentage of the match's duration played as that character. On the character statistics display, however, character wins/losses are rounded to the nearest integer, which is responsible for much of the confusion surrounding the statistic.

As stated by Scott Mercer on the Battle.net forums:

When you get a win, each hero is credited for a portion of the win corresponding to the % of the match's playtime as that hero.

So if you play Mercy 60% of a match you won, and Lucio 40% of that same match, the hero specific win totals on the career overview are credited by 0.6 and 0.4, respectively. We then round the displayed value of games won by hero to the nearest integer.

In Scott's example, assuming 0 wins for both Mercy and Lucio prior to the match, the character statistics would display 1 win for Mercy and 0 for Lucio. The next time a match was won with at least 10% of the game spent playing Lucio, he too would display a win.

The game calculates wins based on what hero you finish the map, as far as my experience goes. In beta, it also seemed to mark all played characters that game as having lost if you didn't win, but I'm not sure if that's still the case.

When I get home later, I'll record my "games won" and "games lost" for a couple of heroes before and after quick play, then update this answer with my findings for some (hopefully) more conclusive evidence to support.