Problem is that we don't create aura::client::WindowTreeClient (and we need to).

Example can be seen in content/shell: https://code.google.com/p/chromium/codesearch#search/&q=WindowTreeClient%20file:%5Esrc/content/shell/&sq=package:chromium&type=cs

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55

commit b6d2d1fcb63abb91d74feb9e0de731e39d83ce55
Author: altimin <altimin@chromium.org>
Date: Thu Apr 21 15:23:22 2016

[headless] Fix crashing during window.open.

* Add WindowTreeClient.
* Register new WebContents in Embedder API.

BUG= 603882 
R=skyostil@chromium.org

Review URL: https://codereview.chromium.org/1891843004

Cr-Commit-Position: refs/heads/master@{#388775}

[modify] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/BUILD.gn
[modify] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/app/headless_shell.cc
[modify] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/lib/browser/headless_browser_impl.cc
[modify] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/lib/browser/headless_browser_impl.h
[modify] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/lib/browser/headless_screen.h
[modify] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/lib/browser/headless_web_contents_impl.cc
[modify] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/lib/browser/headless_web_contents_impl.h
[add] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/lib/browser/headless_window_tree_client.cc
[add] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/lib/browser/headless_window_tree_client.h
[modify] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/lib/headless_browser_browsertest.cc
[modify] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/lib/headless_devtools_client_browsertest.cc
[modify] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/lib/headless_web_contents_browsertest.cc
[modify] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/public/headless_browser.h
[modify] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/public/headless_web_contents.h
[add] https://crrev.com/b6d2d1fcb63abb91d74feb9e0de731e39d83ce55/headless/test/data/window_open.html