Refactor wait_util so it isn't coupled to EarlGrey.

ios/web/public/test/web_view_interaction_test_util.h

 // 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 <UIKit/UIKit.h>
 
 #include <string>
 
 #include "base/ios/block_types.h"
 #include "base/values.h"
 
 #import "ios/web/public/web_state/web_state.h"
 
 namespace web {
 namespace test {
 
-// Synchronously returns the result of executed JavaScript.
+// Synchronously returns the result of executed JavaScript, returning nullptr
+// if the JavaScript does not complete.
 std::unique_ptr<base::Value> ExecuteJavaScript(web::WebState* web_state,
                                                const std::string& script);
 
 // Returns the CGRect, in the coordinate system of web_state's view, that
 // encloses the element with |element_id| in |web_state|'s webview.
 // There is no guarantee that the CGRect returned is inside the current window;
 // callers should check and act accordingly (scrolling the webview, perhaps).
 // Returns CGRectNull if no element could be found.
 CGRect GetBoundingRectOfElementWithId(web::WebState* web_state,
                                       const std::string& element_id);
 
 // Returns whether the element with |element_id| in the passed |web_state| has
 // been tapped using a JavaScript click() event.
 bool TapWebViewElementWithId(web::WebState* web_state,
                              const std::string& element_id);
 
 // Returns whether the element with |element_id| in the passed |web_state| has
 // been focused using a JavaScript focus() event.
 bool FocusWebViewElementWithId(web::WebState* web_state,
                                const std::string& element_id);
 
 // Returns whether the form with |form_id| in the passed |web_state| has been
 // submitted using a JavaScript submit() event.
 bool SubmitWebViewFormWithId(web::WebState* web_state,
                              const std::string& form_id);
 }  // namespace test
 }  // namespace web