Do not update stream synchronization if no new video packet has been received since last update (e.g. video muted).
Do not update stream synchronization if no new video packet has been received since last update (e.g. video muted).

BUG=

Review-Url: https://codereview.webrtc.org/2334113004
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14271}

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