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}
RenderFrameHostImpl* WebContentsImpl::GetFocusedFrame() {
  FrameTreeNode* focused_node = frame_tree_.GetFocusedFrame();
  if (!focused_node)
    return nullptr;
  return focused_node->current_frame_host();
}

RenderFrameHostImpl* WebContentsImpl::GetFocusedFrame() {
  FrameTreeNode* focused_node = frame_tree_.GetFocusedFrame();
  if (!focused_node)
    return nullptr;
  return focused_node->current_frame_host();
}
