Don't call WebContents::DownloadImage() callback if the WebContents were deleted

BUG=583718

Review URL: https://codereview.chromium.org/1685343004

Cr-Commit-Position: refs/heads/master@{#375700}
bool WebContentsImpl::ShouldTransferNavigation() {
  if (!delegate_)
    return true;
  return delegate_->ShouldTransferNavigation();
}

bool WebContentsImpl::ShouldTransferNavigation() {
  if (!delegate_)
    return true;
  return delegate_->ShouldTransferNavigation();
}
