WKWebView 注册js和移除js

/*! @abstract Adds a script message handler to the main world used by page content itself.
@param scriptMessageHandler The script message handler to add.
@param name The name of the message handler.
@discussion Calling this method is equivalent to calling addScriptMessageHandler:contentWorld:name:
with [WKContentWorld pageWorld] as the contentWorld argument.
*/

• (void)addScriptMessageHandler:(id )scriptMessageHandler name:(NSString *)name;
  每调佣一次，webview被retain一次。
  说明这里是强引用，
  在退出的时候，必须remove掉。
  才能彻底释放掉webview.