Fixed CSSVisibilityInterpolationType to store enum instead of double

Fixed ParentVisibilityChecker in CSSVisibilityInterpolationType to store
the EVisibility enum, rather than a double.

Review-Url: https://codereview.chromium.org/2195233002
Cr-Commit-Position: refs/heads/master@{#408908}

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

@@ -95,7 +95,7 @@ private:
         return m_visibility == environment.state().parentStyle()->visibility();
     }
 
-    const double m_visibility;
+    const EVisibility m_visibility;
 };
 
 InterpolationValue CSSVisibilityInterpolationType::createVisibilityValue(EVisibility visibility) const