Add CWVUserContentController which enables injecting JavaScripts.

ios/web_view/public/cwv_user_script.h

+// Copyright 2017 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_PUBLIC_CWV_USER_SCRIPT_H_
+#define IOS_WEB_VIEW_PUBLIC_CWV_USER_SCRIPT_H_
+
+#import <Foundation/Foundation.h>
+
+// User Script to be injected into main frame of CWVWebView after
+// window.document is created, but before other content is loaded (i.e., at the
+// same timing as WKUserScriptInjectionTimeAtDocumentStart).
+@interface CWVUserScript : NSObject
+
+// JavaScript source code.
+@property(nonatomic, copy, readonly, nonnull) NSString* source;
+
+- (nonnull instancetype)init NS_UNAVAILABLE;
+
+- (nonnull instancetype)initWithSource:(nonnull NSString*)source;
+
+@end
+
+#endif  // IOS_WEB_VIEW_PUBLIC_CWV_USER_SCRIPT_H_