Reason for revert:
The new method doesn't work as intended.
It can't pass ownership, because the StatsReports is a vector of raw pointers to StatReport objects owned by the StatsCollector.
Original issue's description:
> New method StatsObserver::OnCompleteReports, passing ownership.
>
> The new name, OnCompleteReports rather than OnComplete, is needed
> because in C++ method lookup, overriding a method hides all otherwise
> inherited methods with the same name, even if they have a different
> signature. And here, the intention is that each subclass should
> override one or the other of the two methods, and inherit the method it
> doesn't override.
>
> This cl is a prerequisite for
> https://codereview.webrtc.org/2567143003/, because the Chrome glue
> code needs to retain the stats report after the OnComplete method has
> returned.
>
> Currently, Chrome makes a copy of the stats mapping (which breaks when
> changing ValuePtr from an rtc::linked_ptr to an std::unique_ptr). After
> this cl, Chrome can be fixed to take ownership and no longer needs to
> copy anything, unblocking cl 2567143003.
>
> BUG=webrtc:6424
>
> Review-Url: https://codereview.webrtc.org/2584553002
> Cr-Commit-Position: refs/heads/master@{#15708}
> Committed: b36ee8d498TBR=solenberg@webrtc.org,magjed@webrtc.org,tkchin@webrtc.org,hbos@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:6424
Review-Url: https://codereview.webrtc.org/2641783002
Cr-Commit-Position: refs/heads/master@{#16144}
e8abe3ef1b