Initialize ios/web_view translate with a system-wide URLRequestContext.

ios/web_view/internal/web_view_web_client.h

 // 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.
 
 #ifndef IOS_WEB_VIEW_INTERNAL_WEB_VIEW_WEB_CLIENT_H_
 #define IOS_WEB_VIEW_INTERNAL_WEB_VIEW_WEB_CLIENT_H_
 
 #include <memory>
 
 #include "base/compiler_specific.h"
 #import "ios/web/public/web_client.h"
 
 namespace ios_web_view {
-class WebViewWebMainParts;
 
 // WebView implementation of WebClient.
 class WebViewWebClient : public web::WebClient {
  public:
   WebViewWebClient();
   ~WebViewWebClient() override;
 
   // WebClient implementation.
   web::WebMainParts* CreateWebMainParts() override;
   std::string GetProduct() const override;
   std::string GetUserAgent(web::UserAgentType type) const override;
   NSString* GetEarlyPageScript(web::BrowserState* browser_state) const override;
 
  private:
-  // The WebMainParts created by |CreateWebMainParts()|.
-  WebViewWebMainParts* web_main_parts_;
-
   DISALLOW_COPY_AND_ASSIGN(WebViewWebClient);
 };
 
 }  // namespace ios_web_view
 
 #endif  // IOS_WEB_VIEW_INTERNAL_WEB_VIEW_WEB_CLIENT_H_