[ElementVisibilityObserver] Fix regression with default threshold

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

Index: third_party/WebKit/Source/core/dom/ElementVisibilityObserver.h
diff --git a/third_party/WebKit/Source/core/dom/ElementVisibilityObserver.h b/third_party/WebKit/Source/core/dom/ElementVisibilityObserver.h
index ef982b86225a44b8e8c7a90d1d210dbcbdce7805..fbc01973b1828a4a824ea8d26a9211abe4cd0db2 100644
--- a/third_party/WebKit/Source/core/dom/ElementVisibilityObserver.h
+++ b/third_party/WebKit/Source/core/dom/ElementVisibilityObserver.h
@@ -5,6 +5,8 @@
 #ifndef ElementVisibilityObserver_h
 #define ElementVisibilityObserver_h
 
+#include <limits>
+
 #include "core/CoreExport.h"
 #include "core/dom/IntersectionObserver.h"
 #include "platform/heap/Heap.h"
@@ -34,7 +36,9 @@ class CORE_EXPORT ElementVisibilityObserver final
   virtual ~ElementVisibilityObserver();
 
   // The |threshold| is the minimum fraction that needs to be visible.
-  void Start(float threshold = 0.0);
+  // See https://github.com/WICG/IntersectionObserver/issues/164 for why this
+  // defaults to std::numeric_limits<float>::min() rather than zero.
+  void Start(float threshold = std::numeric_limits<float>::min());
   void Stop();
 
   void DeliverObservationsForTesting();