Pepper: Access PepperMediaDeviceManager through a WeakPtr

Its lifetime is scoped to the RenderFrame, and it might go away before the
hosts that refer to it.

BUG=423030

Review URL: https://codereview.chromium.org/653243003

Cr-Commit-Position: refs/heads/master@{#299897}
void PepperPlatformVideoCapture::StopCapture() {
  DCHECK(thread_checker_.CalledOnValidThread());
  if (stop_capture_cb_.is_null())
    return;
  stop_capture_cb_.Run();
  stop_capture_cb_.Reset();
}

void PepperPlatformVideoCapture::StopCapture() {
  DCHECK(thread_checker_.CalledOnValidThread());
  if (stop_capture_cb_.is_null())
    return;
  stop_capture_cb_.Run();
  stop_capture_cb_.Reset();
}
