Check that RWHI isn't deleted manually while owned by a scoped_ptr in RVHI

BUG=590284

Review URL: https://codereview.chromium.org/1747183002

Cr-Commit-Position: refs/heads/master@{#378844}
void RenderWidgetHostImpl::LostCapture() {
  if (touch_emulator_)
    touch_emulator_->CancelTouch();

  Send(new InputMsg_MouseCaptureLost(routing_id_));

  if (delegate_)
    delegate_->LostCapture(this);
}

void RenderWidgetHostImpl::LostCapture() {
  if (touch_emulator_)
    touch_emulator_->CancelTouch();

  Send(new InputMsg_MouseCaptureLost(routing_id_));

  if (delegate_)
    delegate_->LostCapture(this);
}
