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}
const GURL& WebContentsImpl::GetMainFrameLastCommittedURL() const {
  return GetLastCommittedURL();
}

const GURL& WebContentsImpl::GetMainFrameLastCommittedURL() const {
  return GetLastCommittedURL();
}
