VCM/Receiver: Only update render time when decoding
VCM/Receiver: Only update render time when decoding

Review URL: https://webrtc-codereview.appspot.com/1336004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: ee184b95209c2dcc2543940d1c7a39f995d077ce

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc