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}
void WebContentsImpl::OnWebContentsDestroyed(WebContentsImpl* web_contents) {
  RemoveDestructionObserver(web_contents);

  for (PendingContents::iterator iter = pending_contents_.begin();
       iter != pending_contents_.end();
       ++iter) {
    if (iter->second != web_contents)
      continue;
    pending_contents_.erase(iter);
    return;
  }
  NOTREACHED();
}

void WebContentsImpl::OnWebContentsDestroyed(WebContentsImpl* web_contents) {
  RemoveDestructionObserver(web_contents);

  for (PendingContents::iterator iter = pending_contents_.begin();
       iter != pending_contents_.end();
       ++iter) {
    if (iter->second != web_contents)
      continue;
    pending_contents_.erase(iter);
    return;
  }
  NOTREACHED();
}
