Add a MathFloor stub generated with TurboFan

src/runtime.js

Index: src/runtime.js
diff --git a/src/runtime.js b/src/runtime.js
index 9d9fe273359e347dd7edc561a87d253b53d69876..247b71ae2744da85f4f37cb4c58231225d1100e1 100644
--- a/src/runtime.js
+++ b/src/runtime.js
@@ -896,7 +896,20 @@ function ToPositiveInteger(x, rangeErrorName) {
    -----------------------------------------------
 */
 
-function STRING_LENGTH_STUB(name) {
-  var receiver = this;  // implicit first parameter
+function StringLengthTF_STUB(receiver, name) {
   return %_StringGetLength(%_JSValueGetValue(receiver));
 }
+
+function MathFloor_STUB(f, i, v) {
+  var r = %_MathFloor(+v);
+  if (%_IsMinusZero(r)) {
+    %_FixedArraySet(%_GetTypeFeedbackVector(f), ((i|0)+1)|0, 1);
+    return -0;
+  }
+  // Return integers in smi range as smis.
+  var trunc = r|0;
+  if (trunc === r) {
+    return trunc;
+  }
+  return r;
+}