[ios] Use new JS execution API in browsing data partitioning test.

ios/web/public/test/js_test_util.mm

 // Copyright 2014 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/js_test_util.h"
 
 #import <WebKit/WebKit.h>
 
 #import "base/logging.h"
 #import "base/mac/scoped_nsobject.h"
@@ skipping 19 matching lines @@
   return [[result retain] autorelease];
 }
 
 id ExecuteJavaScript(CRWJSInjectionReceiver* receiver, NSString* script) {
   base::scoped_nsobject<CRWJSInjectionManager> manager(
       [[CRWJSInjectionManager alloc] initWithReceiver:receiver]);
   return ExecuteJavaScript(manager, script);
 }
 
 id ExecuteJavaScript(WKWebView* web_view, NSString* script) {
+  return ExecuteJavaScript(web_view, script, nullptr);
+}
+
+id ExecuteJavaScript(WKWebView* web_view, NSString* script, NSError** error) {
   __block base::scoped_nsobject<id> result;
   __block bool completed = false;
   [web_view evaluateJavaScript:script
-             completionHandler:^(id evaluation_result, NSError* error) {
-               DCHECK(!error);
-               result.reset([evaluation_result copy]);
+             completionHandler:^(id script_result, NSError* script_error) {
+               result.reset([script_result copy]);
+               if (error)
+                 *error = [[script_error copy] autorelease];
                completed = true;
              }];
   base::test::ios::WaitUntilCondition(^{
     return completed;
   });
   return [[result retain] autorelease];
 }
 
 }  // namespace web