Codify the assumption that %GetArrayKeys can return only a single interval starting at zero

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index d9dd12e1876967373184d057fe465647a141e2c2..56dcd14c1785a843628476dcac3c7cb457bfca76 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -9569,9 +9569,8 @@ static Handle<Object> NewSingleInterval(Isolate* isolate, uint32_t length) {
 
 
 // Returns an array that tells you where in the [0, length) interval an array
-// might have elements.  Can either return keys (positive integers) or
-// intervals (pair of a negative integer (-start-1) followed by a
-// positive (length)) or undefined values.
+// might have elements.  Can either return keys (positive integers or undefined)
+// or a single interval (pair of -1, positive length).
 // Intervals can span over some keys that are not in the object.
 RUNTIME_FUNCTION(MaybeObject*, Runtime_GetArrayKeys) {
   HandleScope scope(isolate);