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