core(driver): add driver.wsEndpoint() (#3864)

lighthouse-core/gather/connections/connection.js

@@ -31,6 +31,15 @@ class Connection {
     return Promise.reject(new Error('Not implemented'));
   }
 
+
+  /**
+   * @return {!Promise<string>}
+   */
+  wsEndpoint() {
+    return Promise.reject(new Error('Not implemented'));
+  }
+
+
   /**
    * Call protocol methods
    * @param {!string} method

lighthouse-core/gather/connections/cri.js

@@ -139,6 +139,15 @@ class CriConnection extends Connection {
     });
   }
 
+  /**
+   * @override
+   * @return {!Promise<string>}
+   */
+  wsEndpoint() {
+    return this._runJsonCommand('version').then(response => response.webSocketDebuggerUrl);
+  }
+
+
   /**
    * @override
    * @param {string} message

lighthouse-core/gather/driver.js

@@ -105,6 +105,15 @@ class Driver {
     return this._connection.disconnect();
   }
 
+  /**
+   * Get the browser WebSocket endpoint for devtools protocol clients like Puppeteer.
+   * Only works with WebSocket connection, not extension or devtools.
+   * @return {!Promise<string>}
+   */
+  wsEndpoint() {
+    return this._connection.wsEndpoint();
+  }
+
   /**
    * Bind listeners for protocol events
    * @param {!string} eventName