Allow webrtc::test::VcmCapturer to be updated with its current sink.
Allow webrtc::test::VcmCapturer to be updated with its current sink.

BUG=none

Review-Url: https://codereview.webrtc.org/2470393002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14967}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: b5b0b98f588de2e559f89752eab88f52c7e3f1d2

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