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::OnBrowserPluginMessage(RenderFrameHost* render_frame_host,
                                             const IPC::Message& message) {
  CHECK(!browser_plugin_embedder_.get());
  CreateBrowserPluginEmbedderIfNecessary();
  browser_plugin_embedder_->OnMessageReceived(message, render_frame_host);
}

void WebContentsImpl::OnBrowserPluginMessage(RenderFrameHost* render_frame_host,
                                             const IPC::Message& message) {
  CHECK(!browser_plugin_embedder_.get());
  CreateBrowserPluginEmbedderIfNecessary();
  browser_plugin_embedder_->OnMessageReceived(message, render_frame_host);
}
