Make CreateRtcLogStreamConfig() return a std::unique_ptr

This is done for two reasons:
1. This will be useful in an upcoming CL, when all logging will be done with one function that accepts a std::unique_ptr<RtcEvent>.
2. Ensure no unnecessary copies when the compiler fails to optimize.

BUG=webrtc:8111
TBR=stefan@webrtc.org

Review-Url: https://codereview.webrtc.org/3010273002
Cr-Original-Commit-Position: refs/heads/master@{#19746}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8ec568a9ff9b30dd7e8881a0c1bdbc418dc05ff4
1 file changed