Change iOS web matcher argument types to std::string.

ios/web/public/test/earl_grey/web_view_matchers.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 <string>
+
 #import <EarlGrey/EarlGrey.h>
 
 #import "ios/web/public/web_state/web_state.h"
 
 namespace web {
 
+// TODO(crbug.com/614167): Remove this method when it is no longer used.
 // Shorthand for GREYMatchers::matcherForWebViewContainingText:inWebState.
 id<GREYMatcher> webViewContainingText(NSString* text, web::WebState* webState);
 
+// Shorthand for GREYMatchers::matcherForWebViewContainingText:inWebState.
+id<GREYMatcher> webViewContainingText(const std::string& text,
+                                      web::WebState* webState);
+
 }  // namespace web
 
 @interface GREYMatchers (WebViewAdditions)
 
 // Matcher for WKWebView containing |text|.
-+ (id<GREYMatcher>)matcherForWebViewContainingText:(NSString*)text
++ (id<GREYMatcher>)matcherForWebViewContainingText:(const std::string&)text
                                         inWebState:(web::WebState*)webState;
 
 @end