Change JitterBuffer::GetNackList to return a std::vector<uint16_t>.

This fixed the problem with returning a pointer to an internal buffer
of a JitterBuffer.

R=stefan@webrtc.org
BUG=none
TEST=none

Review URL: https://webrtc-codereview.appspot.com/53639004

Cr-Original-Commit-Position: refs/heads/master@{#9365}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: b1825a40380a1339de713efcebdb8c1d6bfeb173
8 files changed