Removed unused async_invoker_ in WebRtcVideoCapturer

BUG=None

Review-Url: https://codereview.webrtc.org/3001443002
Cr-Original-Commit-Position: refs/heads/master@{#19333}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f3238e43ce36187ff57dcb7a66131809157697cb
2 files changed