Add warning if result of Object::ToArrayIndex is unused

Change-Id: Id50564cc1a83de218fb30293bed3eb0a8fcdd8c0
Reviewed-on: https://chromium-review.googlesource.com/c/1283011
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56824}

src/objects.h

@@ -1382,7 +1382,7 @@ class Object {
   // Tries to convert an object to an array index. Returns true and sets the
   // output parameter if it succeeds. Equivalent to ToArrayLength, but does not
   // allow kMaxUInt32.
-  inline bool ToArrayIndex(uint32_t* index) const;
+  V8_WARN_UNUSED_RESULT inline bool ToArrayIndex(uint32_t* index) const;
 
   // Returns true if the result of iterating over the object is the same
   // (including observable effects) as simply accessing the properties between 0