[IntersectionObserver] Enable observations across shadow DOM roots.

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

Index: third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
diff --git a/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp b/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
index 9cee27481920c954a429a4630b7aa65ac108cd3c..1970c981dc1e8de5d0fe50185daa80c0d5569d65 100644
--- a/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
+++ b/third_party/WebKit/Source/core/dom/IntersectionObserver.cpp
@@ -249,7 +249,7 @@ void IntersectionObserver::observe(Element* target, ExceptionState& exceptionSta
 
     if (target->document() == rootNode()->document()) {
         shouldReportRootBounds = true;
-        isDOMDescendant = target->isDescendantOf(rootNode());
+        isDOMDescendant = rootNode()->isShadowIncludingInclusiveAncestorOf(target);
     } else if (targetFrame && rootFrame) {
         shouldReportRootBounds = targetFrame->securityContext()->getSecurityOrigin()->canAccess(rootFrame->securityContext()->getSecurityOrigin());
         isDOMDescendant = (targetFrame->tree().top() == rootFrame);