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 PageImportanceSignals& WebContentsImpl::GetPageImportanceSignals() const {
  return page_importance_signals_;
}

const PageImportanceSignals& WebContentsImpl::GetPageImportanceSignals() const {
  return page_importance_signals_;
}
