IntersectionObserver: notify when root or target is removed.

third_party/WebKit/Source/core/dom/IntersectionObserver.cpp

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/dom/IntersectionObserver.h"
 
 #include "bindings/core/v8/ExceptionState.h"
 #include "core/css/parser/CSSParserTokenRange.h"
 #include "core/css/parser/CSSTokenizer.h"
 #include "core/dom/Element.h"
@@ skipping 176 matching lines @@
 {
     if (!target || !target->intersectionObserverData())
         return;
     // TODO(szager): unobserve callback
     if (IntersectionObservation* observation = target->intersectionObserverData()->getObservationFor(*this))
         observation->disconnect();
 }
 
 void IntersectionObserver::computeIntersectionObservations()
 {
-    if (!m_root || !m_root->inShadowIncludingDocument())

[ojan, 2016/04/15 18:19:08]

Moving this makes me a bit sad because it means we potentially do a lot of work
for an observer whose root is not in the tree, but I see why this patch needs
it. I think it's fine though. If we see code in the wild doing this, we can
optimize it (e.g. only do this work the first time the root is removed from the
tree instead of each frame).

-        return;
     Document* callbackDocument = toDocument(m_callback->getExecutionContext());
     if (!callbackDocument)
         return;
     LocalDOMWindow* callbackDOMWindow = callbackDocument->domWindow();
     if (!callbackDOMWindow)
         return;
     DOMHighResTimeStamp timestamp = DOMWindowPerformance::performance(*callbackDOMWindow)->now();
     for (auto& observation : m_observations)
         observation->computeIntersectionObservations(timestamp);
 }
@@ skipping 97 matching lines @@
 
 DEFINE_TRACE(IntersectionObserver)
 {
     visitor->template registerWeakMembers<IntersectionObserver, &IntersectionObserver::clearWeakMembers>(this);
     visitor->trace(m_callback);
     visitor->trace(m_observations);
     visitor->trace(m_entries);
 }
 
 } // namespace blink