The patch for https://bugs.webkit.org/show_bug.cgi?id=84493 disabled the collection of elements from shadow tree into a flow thread. It should be possible to do that properly. Please take into account the CSSOM part when solving this bug.
Created corresponding Blink issue over at https://code.google.com/p/chromium/issues/detail?id=243276
Closing as WONTFIX since the ShadowDOM code has been removed from WebKit