Enable -Wundeclared-selector on ios.

ios/web/test/wk_web_view_crash_utils.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/test/wk_web_view_crash_utils.h"
 
 #import <Foundation/Foundation.h>
 #import <WebKit/WebKit.h>
 
 #include "base/logging.h"
@@ skipping 21 matching lines @@
 
 }  // namespace
 
 namespace web {
 
 void SimulateWKWebViewCrash(WKWebView* webView) {
   SEL selector = @selector(webViewWebContentProcessDidTerminate:);
   if ([webView.navigationDelegate respondsToSelector:selector]) {
     [webView.navigationDelegate performSelector:selector withObject:webView];
   }
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wundeclared-selector"
   [webView performSelector:@selector(_processDidExit)];
+#pragma clang diagnostic pop
 }
 
 WKWebView* BuildTerminatedWKWebView() {
   id fail = ^void(NSInvocation* invocation) {
       // Always fails with WKErrorWebContentProcessTerminated error.
       NSError* error =
           [NSError errorWithDomain:WKErrorDomain
                               code:WKErrorWebContentProcessTerminated
                           userInfo:nil];
 
       void (^completionHandler)(id, NSError*) =
           [invocation getArgumentAtIndexAsObject:3];
       completionHandler(nil, error);
   };
   return BuildMockWKWebViewWithStubbedJSEvalFunction(fail);
 }
 
 WKWebView* BuildHealthyWKWebView() {
   id succeed = ^void(NSInvocation* invocation) {
       void (^completionHandler)(id, NSError*) =
           [invocation getArgumentAtIndexAsObject:3];
       // Always succceeds with nil result.
       completionHandler(nil, nil);
   };
   return BuildMockWKWebViewWithStubbedJSEvalFunction(succeed);
 }
 
 }  // namespace web