Modify the parameter type of PeerConnectionObserver callback OnAddTrack.

Change the second parameter type to a const reference of vector so that
the vector will not be copied.

BUG=none

Review-Url: https://codereview.webrtc.org/2551603003
Cr-Original-Commit-Position: refs/heads/master@{#15396}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: c63b89468682eef761a6d444d1fd6445d7a624ed
2 files changed