Add root margin support for IntersectionObserver.

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

Index: third_party/WebKit/Source/core/dom/IntersectionObserver.h
diff --git a/third_party/WebKit/Source/core/dom/IntersectionObserver.h b/third_party/WebKit/Source/core/dom/IntersectionObserver.h
index 34a89d2615172d6f5baee6413d314695c37434e9..9072dde26eeabe8fa9b92f57df16bb469db5dcbd 100644
--- a/third_party/WebKit/Source/core/dom/IntersectionObserver.h
+++ b/third_party/WebKit/Source/core/dom/IntersectionObserver.h
@@ -34,9 +34,15 @@ public:
 
     Element* root() { return m_root.get(); }
     LayoutObject* rootLayoutObject();
+    bool hasPercentMargin() const;
+    const Length& topMargin() const { return m_topMargin; }
+    const Length& rightMargin() const { return m_rightMargin; }
+    const Length& bottomMargin() const { return m_bottomMargin; }
+    const Length& leftMargin() const { return m_leftMargin; }
     bool isDescendantOfRoot(const Element*) const;
     void computeIntersectionObservations(double timestamp);
     void enqueueIntersectionObserverEntry(IntersectionObserverEntry&);
+    void applyRootMargin(LayoutRect&) const;
     unsigned firstThresholdGreaterThan(float ratio) const;
     void deliver();
     void setActive(bool);
@@ -46,7 +52,7 @@ public:
     DECLARE_TRACE();
 
 private:
-    explicit IntersectionObserver(IntersectionObserverCallback&, Element&, const Vector<float>& thresholds);
+    explicit IntersectionObserver(IntersectionObserverCallback&, Element&, const Vector<Length>& rootMargin, const Vector<float>& thresholds);
 
     void checkRootAndDetachIfNeeded();
 
@@ -55,6 +61,10 @@ private:
     HeapHashSet<WeakMember<IntersectionObservation>> m_observations;
     HeapVector<Member<IntersectionObserverEntry>> m_entries;
     Vector<float> m_thresholds;
+    Length m_topMargin;
+    Length m_rightMargin;
+    Length m_bottomMargin;
+    Length m_leftMargin;

[ojan, 2016/01/12 01:08:49]

This object is getting quite large. I don't think we need to do anything about
it at the moment, but when we think about perf/memory profiling, we should
consider the case that there are a lot of observers on the page. We might want
to move margins and thresholds to a rare data object in that case. 

[szager1, 2016/01/12 19:24:43]

Acknowledged.  I would expect that the number of observers would pretty much
always be small, although the number of observations might be much higher.  But
hard to predict how this might be used in the wild.

 };
 
 } // namespace blink