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::OnFirstPaintAfterLoad() {
  if (new_content_rendering_timeout_->IsRunning()) {
    new_content_rendering_timeout_->Stop();
  } else {
    received_paint_after_load_ = true;
  }
}

void RenderWidgetHostImpl::OnFirstPaintAfterLoad() {
  if (new_content_rendering_timeout_->IsRunning()) {
    new_content_rendering_timeout_->Stop();
  } else {
    received_paint_after_load_ = true;
  }
}
