Don't reattach the host when adding a new ShadowRoot 

We don't need to reattach the host when adding a new ShadowRoot, we only 
need to reattach the ShadowRoots and mark the host for a style recalc. 

The one exception to this is apparently <input> where swapping the 
InputTypeView requires a reattach. Some day it might be nice to fix that, 
but for now the common case works. 

This lets you add a ShadowRoot to an element that is currently animating 
without stopping the animation resolving the very old bug: 
https://bugs.webkit.org/show_bug.cgi?id=94905 where tasak@ noticed the issue.

In fixing this I also realized that my code in ChildNodeInsertionNotifier 
was wrong since it used parentNode() instead of parentOrShadowHostNode() 
which meant that ShadowRoots would not get lazyAttached properly by the 
notifier, and the only reason they were getting attached was because we
lazyReattached the host inside ElementShadow::addShadowRoot.

Review URL: https://codereview.chromium.org/70153002

git-svn-id: svn://svn.chromium.org/blink/trunk@161896 bbb929c8-8fbe-4397-9dbb-9b2b20218538