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}
WebContents* WebContents::FromRenderFrameHost(RenderFrameHost* rfh) {
  if (!rfh)
    return nullptr;
  return static_cast<RenderFrameHostImpl*>(rfh)->delegate()->GetAsWebContents();
}

WebContents* WebContents::FromRenderFrameHost(RenderFrameHost* rfh) {
  if (!rfh)
    return nullptr;
  return static_cast<RenderFrameHostImpl*>(rfh)->delegate()->GetAsWebContents();
}
