[builtins] Fix String.prototype.indexOf with negative positions

src/builtins/builtins-string.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/builtins/builtins-utils.h"
 #include "src/builtins/builtins.h"
 #include "src/code-factory.h"
 #include "src/code-stub-assembler.h"
 #include "src/regexp/regexp-utils.h"
 
@@ skipping 888 matching lines @@
     search_string.Bind(arguments.AtIndex(0));
     position.Bind(SmiConstant(0));
     Goto(&fast_path);
   }
   Bind(&argc_2);
   {
     Comment("2 Argument case");
     search_string.Bind(arguments.AtIndex(0));
     position.Bind(arguments.AtIndex(1));
     GotoUnless(TaggedIsSmi(position.value()), &call_runtime);
+    position.Bind(SmiMax(position.value(), SmiConstant(0)));
     Goto(&fast_path);
   }
 
   Bind(&fast_path);
   {
     Comment("Fast Path");
     Label zero_length_needle(this);
     GotoIf(TaggedIsSmi(receiver), &call_runtime);
     Node* needle = search_string.value();
     GotoIf(TaggedIsSmi(needle), &call_runtime);
@@ skipping 694 matching lines @@
         Runtime::kThrowIncompatibleMethodReceiver, context,
         assembler.HeapConstant(assembler.factory()->NewStringFromAsciiChecked(
             "String Iterator.prototype.next", TENURED)),
         iterator);
     assembler.Return(result);  // Never reached.
   }
 }
 
 }  // namespace internal
 }  // namespace v8