Add unit tests to ReceiveStatisticsProxy class.
Add unit tests to ReceiveStatisticsProxy class.

BUG=none

Review-Url: https://codereview.webrtc.org/2450363003
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14911}

ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc