Fix a comparison with negative 0.

No difference in behavior because the other branch of the condition
already allowed positive 0.

Change-Id: Ia31d3366f882b5eaf004f17d26e4213f8de57794
Reviewed-on: https://chromium-review.googlesource.com/c/1435936Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59111}

src/builtins/builtins-number.cc

@@ -225,7 +225,7 @@ BUILTIN(NumberPrototypeToString) {
 
   // Fast case where the result is a one character string.
   if ((IsUint32Double(value_number) && value_number < radix_number) ||
-      value_number == -0.0) {
+      IsMinusZero(value_number)) {
     // Character array used for conversion.
     static const char kCharTable[] = "0123456789abcdefghijklmnopqrstuvwxyz";
     return *isolate->factory()->LookupSingleCharacterStringFromCode(