[ios] Fixed memory management in ExecuteScriptOnInterstitial.

ios/web/public/test/earl_grey/js_test_util.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/web/public/test/earl_grey/js_test_util.h"
 
 #import <EarlGrey/EarlGrey.h>
 #import <WebKit/WebKit.h>
 
 #include "base/timer/elapsed_timer.h"
@@ skipping 66 matching lines @@
   return [result autorelease];
 }
 
 id ExecuteScriptOnInterstitial(WebState* web_state, NSString* script) {
   web::WebInterstitialImpl* interstitial =
       static_cast<web::WebInterstitialImpl*>(web_state->GetWebInterstitial());
 
   __block id script_result = nil;
   __block bool did_finish = false;
   web::ExecuteScriptForTesting(interstitial, script, ^(id result, NSError*) {
-    script_result = [[result copy] autorelease];
+    script_result = [result copy];
     did_finish = true;
   });
   BOOL suceeded = WaitUntilConditionOrTimeout(kWaitForJSCompletionTimeout, ^{
     return did_finish;
   });
   GREYAssert(suceeded, @"Script execution timed out");
-  return script_result;
+  return [script_result autorelease];
 }
 
 }  // namespace web