Fix crash when registering abs-send-time to AudioSend/ReceiveStream.

Introduced with r14870.

BUG=b/32591921

Review-Url: https://codereview.webrtc.org/2473663002
Cr-Original-Commit-Position: refs/heads/master@{#14883}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 572ae1212b488d874f6afcf87bc536319b122df7
2 files changed