Add a use-counter for viewport-fixed position:fixed under a filter

third_party/WebKit/Source/core/frame/UseCounter.h

Index: third_party/WebKit/Source/core/frame/UseCounter.h
diff --git a/third_party/WebKit/Source/core/frame/UseCounter.h b/third_party/WebKit/Source/core/frame/UseCounter.h
index e0208cc4f666ba802f3cdb70bbab11164a198233..4ab04349a5d020305b9c2a951d3a22f90f83711d 100644
--- a/third_party/WebKit/Source/core/frame/UseCounter.h
+++ b/third_party/WebKit/Source/core/frame/UseCounter.h
@@ -1574,6 +1574,7 @@ class CORE_EXPORT UseCounter {
     kMediaStreamConstraintsGoogDAEchoCancellation = 1958,
     kMediaStreamConstraintsGoogNoiseReduction = 1959,
     kMediaStreamConstraintsGoogPowerLineFrequency = 1960,
+    kViewportFixedPositionUnderFilter = 1961,
 
     // Add new features immediately above this line. Don't change assigned
     // numbers of any item, and don't reuse removed slots.