Change FrameBuffer::Stop to not require a critical section.

BUG=webrtc:7331

Review-Url: https://codereview.webrtc.org/2749563002
Cr-Original-Commit-Position: refs/heads/master@{#17228}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 0a735643388622494cd59720a40b51899985be99
2 files changed