Add a MathFloor stub generated with TurboFan

src/interface-descriptors.h

Index: src/interface-descriptors.h
diff --git a/src/interface-descriptors.h b/src/interface-descriptors.h
index ab1517e0df6400bd7805bb2c31f8f63be3758e7a..6f69f3c8ceb7c32071522b69b88a8cb5f90c9273 100644
--- a/src/interface-descriptors.h
+++ b/src/interface-descriptors.h
@@ -58,6 +58,7 @@ class PlatformInterfaceDescriptor;
   V(StoreArrayLiteralElement)                 \
   V(MathPowTagged)                            \
   V(MathPowInteger)                           \
+  V(MathRoundVariant)                         \
   V(ContextOnly)                              \
   V(GrowArrayElements)
 
@@ -528,6 +529,12 @@ class MathPowIntegerDescriptor : public CallInterfaceDescriptor {
 };
 
 
+class MathRoundVariantDescriptor : public CallInterfaceDescriptor {
+ public:
+  DECLARE_DESCRIPTOR(MathRoundVariantDescriptor, CallInterfaceDescriptor)
+};
+
+
 class ContextOnlyDescriptor : public CallInterfaceDescriptor {
  public:
   DECLARE_DESCRIPTOR(ContextOnlyDescriptor, CallInterfaceDescriptor)