Added ToString functions in a separate header and move the ostream
operators to a test only header.
Bug: webrtc:8982
Change-Id: If547173aa39bbae2244531e2d3091886f14eae31
Reviewed-on: https://webrtc-review.googlesource.com/65280
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Jonas Olsson <jonasolsson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22674}