[ElementVisibilityObserver] Fix regression with default threshold

https://codereview.chromium.org/2830713003 (r468137,
913ee5fe) changed the default threshold
of ElementVisibilityObserver from std::numeric_limits<float>::min() to
zero, assuming that they would behave the same.

It turns out that there's some confusion here - see
https://github.com/WICG/IntersectionObserver/issues/164 - and for now
we should continue to use std::numeric_limits<float>::min() to
distinguish between completely hidden and partially visible.

BUG=726839

Review-Url: https://codereview.chromium.org/2919543002
Cr-Commit-Position: refs/heads/master@{#476265}

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

@@ -52,7 +52,7 @@ DEFINE_TRACE(ElementVisibilityObserver) {
 
 void ElementVisibilityObserver::OnVisibilityChanged(
     const HeapVector<Member<IntersectionObserverEntry>>& entries) {
-  bool is_visible = entries.back()->intersectionRatio() >
+  bool is_visible = entries.back()->intersectionRatio() >=
                     intersection_observer_->thresholds()[0];
   (*callback_.get())(is_visible);
 }

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();