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::ReplicatePageFocus(bool is_focused) {
  if (is_being_destroyed_)
    return;

  frame_tree_.ReplicatePageFocus(is_focused);
}

void WebContentsImpl::ReplicatePageFocus(bool is_focused) {
  if (is_being_destroyed_)
    return;

  frame_tree_.ReplicatePageFocus(is_focused);
}
