IntersectionObserver: use nullptr for implicit root.

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

Index: third_party/WebKit/Source/core/dom/IntersectionObservation.cpp
diff --git a/third_party/WebKit/Source/core/dom/IntersectionObservation.cpp b/third_party/WebKit/Source/core/dom/IntersectionObservation.cpp
index 724c4312e55bd32fe530d9023a8e2c4d92af009c..051e8b1055ff2b01e45d89f765d8d9da7118dced 100644
--- a/third_party/WebKit/Source/core/dom/IntersectionObservation.cpp
+++ b/third_party/WebKit/Source/core/dom/IntersectionObservation.cpp
@@ -27,10 +27,8 @@ void IntersectionObservation::computeIntersectionObservations(
   rootMargin[1] = m_observer->rightMargin();
   rootMargin[2] = m_observer->bottomMargin();
   rootMargin[3] = m_observer->leftMargin();
-  Node* rootNode = m_observer->rootNode();
-  IntersectionGeometry geometry(
-      rootNode && !rootNode->isDocumentNode() ? toElement(rootNode) : nullptr,
-      *target(), rootMargin, m_shouldReportRootBounds);
+  IntersectionGeometry geometry(m_observer->root(), *target(), rootMargin,
+                                m_shouldReportRootBounds);

[kenrb, 2016/12/09 21:01:43]

There is something I don't quite understand here. m_observer->root() can now
return nullptr, but IntersectionGeometry::computeGeometry() on the next line
will always return early if it has a null root. Is this intentional? How do we
get intersection calculations in that case?

[szager1, 2016/12/10 20:51:18]

This patch depends on another patch (https://codereview.chromium.org/2553103004)
that fixes IntersectionGeometry::computeGeometry() to treat null as the implicit
root.

   geometry.computeGeometry();
 
   // Some corner cases for threshold index: