Don't animate visibility keyframes that don't include "visible"

third_party/WebKit/Source/core/animation/CSSVisibilityInterpolationType.cpp

Index: third_party/WebKit/Source/core/animation/CSSVisibilityInterpolationType.cpp
diff --git a/third_party/WebKit/Source/core/animation/CSSVisibilityInterpolationType.cpp b/third_party/WebKit/Source/core/animation/CSSVisibilityInterpolationType.cpp
index fa1abf6b151d276bbd55e1852d7b310f3a6a0c8d..008414c7d7df9f7c8e130937e963056f6e48b30d 100644
--- a/third_party/WebKit/Source/core/animation/CSSVisibilityInterpolationType.cpp
+++ b/third_party/WebKit/Source/core/animation/CSSVisibilityInterpolationType.cpp
@@ -30,9 +30,8 @@ class CSSVisibilityNonInterpolableValue : public NonInterpolableValue {
       return m_start;
     if (fraction >= 1)
       return m_end;
-    if (m_start == EVisibility::kVisible || m_end == EVisibility::kVisible)
-      return EVisibility::kVisible;
-    return fraction < 0.5 ? m_start : m_end;
+    DCHECK(m_start == EVisibility::kVisible || m_end == EVisibility::kVisible);
+    return EVisibility::kVisible;
   }
 
   DECLARE_NON_INTERPOLABLE_VALUE_TYPE();
@@ -161,13 +160,23 @@ CSSVisibilityInterpolationType::maybeConvertStandardPropertyUnderlyingValue(
 PairwiseInterpolationValue CSSVisibilityInterpolationType::maybeMergeSingles(
     InterpolationValue&& start,
     InterpolationValue&& end) const {
-  return PairwiseInterpolationValue(
-      InterpolableNumber::create(0), InterpolableNumber::create(1),
-      CSSVisibilityNonInterpolableValue::create(
-          toCSSVisibilityNonInterpolableValue(*start.nonInterpolableValue)
-              .visibility(),
-          toCSSVisibilityNonInterpolableValue(*end.nonInterpolableValue)
-              .visibility()));
+  EVisibility startVisibility =
+      toCSSVisibilityNonInterpolableValue(*start.nonInterpolableValue)
+          .visibility();
+  EVisibility endVisibility =
+      toCSSVisibilityNonInterpolableValue(*end.nonInterpolableValue)
+          .visibility();
+  // One side must be "visible".
+  // Spec: https://drafts.csswg.org/css-transitions/#animtype-visibility
+  if (startVisibility != endVisibility &&
+      startVisibility != EVisibility::kVisible &&
+      endVisibility != EVisibility::kVisible) {
+    return nullptr;
+  }
+  return PairwiseInterpolationValue(InterpolableNumber::create(0),
+                                    InterpolableNumber::create(1),
+                                    CSSVisibilityNonInterpolableValue::create(
+                                        startVisibility, endVisibility));
 }
 
 void CSSVisibilityInterpolationType::composite(