[debug] Fix SloppyArgumentsElements verifier.

BUG=chromium:713365

Change-Id: I5d9d5b5e00a637923a1a3e0dc7f81fa4075c4e82
Reviewed-on: https://chromium-review.googlesource.com/484300Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44782}

src/objects-debug.cc

@@ -495,8 +495,9 @@ void SloppyArgumentsElements::SloppyArgumentsElementsVerify(
     JSSloppyArgumentsObject* holder) {
   Isolate* isolate = GetIsolate();
   FixedArrayVerify();
-  // Abort verification if only partially initialized.
-  if (arguments()->IsUndefined(isolate)) return;
+  // Abort verification if only partially initialized (can't use arguments()
+  // getter because it does FixedArray::cast()).
+  if (get(kArgumentsIndex)->IsUndefined(isolate)) return;
 
   ElementsKind kind = holder->GetElementsKind();
   CHECK(IsFixedArray());