Don't use ChildNodeRemovalNotifier for ShadowRoot removal.

As ShadowRoot is removed only from ElementShadow destructor,
its subtree removal should be treated as "detached"
thus use removeDetachedChildren(), otherwise  removedFrom()
implementations will encounter inconsistent  state
which isn't updated on parent's "detached" removal.

BUG=331574
TEST=w3c/web-platform-tests/shadow-dom/shadow-trees/upper-boundary-encapsulation/*
R=esprehn, adamk

Review URL: https://codereview.chromium.org/127733002

git-svn-id: svn://svn.chromium.org/blink/trunk@164681 bbb929c8-8fbe-4397-9dbb-9b2b20218538